:: Your argument about time is weak, at best, for at least two reasons:
: Based upon Revelation 2, I'll point out the following as possible scenarios whereby there is no basis for belief that God would overlook false prophesying:
Fair enough. Let's take a closer look.
: Any false teaching deviant from known truth which persists beyond the decades equivalent to that between the forming of the Christian community and the writing of Revelation. (Which I presume to be not quite a full century.)
If we accept WTS dates for these events, 33 and about 96 C.E., we get about 63 years as an outside limit.
: Any false teaching deviant from known truth which persists several decades after being thoroughly proven wrong.
That could mean anything from 20 years on up.
Let's note that, as usual, you've built a lot of fuzziness into your definitions. We don't really know how long "several decades" amounts to, so you've left yourself a lot of latitude to change your tune later on as people try to tie you down to specifics. But the biggest sources of fuzz are the notions of being "deviant from known truth" and "being thoroughly proven wrong". One can always claim, with or without real foundation, that nothing at all has ever been "thoroughly established". All we have to do to find examples is to look at discussions of chronology in WTS literature or from thorough-going crackpots like Gary La Motta.
So, to nail things down better, I'll take the position that something has been "thoroughly proven" when a large majority of secular scholars have accepted something for at least 20 years.
: From my two expressions above you can tell that it's not so much what is taught, who teaches it or even how long they have been teaching it, but rather how and when they react once error has been established. IMO, the fact that the WTS has been willing to change understandings has been to their favor, even if this was imposed upon them.
Sure, being willing to change when they're proved wrong is in their favor. But much of the time their willingness has been along the lines of one's making grandiose claims to be able to fly, then crashing and admitting in the intensive care unit that one couldn't fly after all.
A much better test of humility, and of being willing to change, is to accept constructive criticism and solid arguments that intelligent people give before one enters the crash and burn stage. The Society has not done this often. Another test is not to disfellowship those who publicly air constructive criticism.
:: The Society's leaders have been teaching some major false doctrines for about 120 years.
: The same exact teachings or variations of some idea? Or, have they been willing to change as they realized aspects of teachings were incorrect?
This is another pile of fuzz you've built into your litmus test. Since the Society changes its doctrines like I change my underwear, there's not much that hasn't undergone a certain amount of change in 120 years. But a number of basics have remained roughly the same, and those are sufficient to prove my case. In other words, things like changing the initial date for Christ's parousia from 1874 to 1914 count as minor variations.
Having looked at some caveats, let's see whether the Society fails your tests:
The WTS originally taught that Christ returned invisibly in 1874, and taught a number of auxiliary doctrines based on this date. During the 1920s to 1940s, these were gradually modified to hinge on 1914.
The 1914 date has been taught to be extremely important in "Bible chronology" from the beginning, although what the date meant has changed drastically except for the label "end of the gentile times". This "gentile times" chronology has been thoroughly proven incorrect by secular scholars since at least as far back as 1900, since the Society's date system is inconsistent with archaeological and historical findings.
Specifically, the Society has claimed that Jerusalem was destroyed in 606/7 B.C.E., while historians have held since about 1900 (some as far back as about 1850) that it was destroyed in 586/7 B.C.E. The WTS only began to accept some solid historical findings around 1943, when it changed the beginning of the "gentile times" from 606 to 607 B.C.E. In 1944 the WTS changed the date for Jerusalem's destruction to 607 B.C.E. Both changes were given in explanations that amounted to gross lies, for which the WTS has never apologized, and has never even acknowledged.
In the 1970s several people published scholarly treatises showing that WTS chronology was wrong. The WTS never dealt with this material.
In 1977 a JW named Carl Olof Jonsson sent to the WTS in Brooklyn a pile of research material that proved conclusively that WTS chronology was in error. Jonsson was disfellowshiped for his efforts. In 1983 Jonsson published a more complete version of his findings in the book The Gentile Times Reconsidered. The Society never dealt with any of the material in the book or in the 2nd edition published about 1985. The Society's writers are quite familiar with the material, however.
So here we have a case where a major WTS doctrine has been published in various forms for some 120 years. Depending on how one likes to measure, the WTS has been aware for between 20 and 100 years that various aspects of this doctrine are wrong. It has failed to correct its teaching.
So, Friend, we have an example where the Society fails both of your tests. You yourself have admitted that you reject this WTS chronology, so you cannot argue that their teaching has not been thoroughly disproved.
2. View of Itself as God's Channel
C. T. Russell began to view himself as God's unique channel to mankind even before he incorporated the Watchtower Society in 1884. He and his successors have viewed themselves and/or their organization as "God's channel" ever since.
The present day WTS rejects at least 90% of what Russell taught, so that even it ought to admit that Russell was not "God's channel" in any meaningful sense. But it does not. It still teaches that Russell was indeed this "channel", and that God successively refined later "channels". It has never explained how anyone could be "God's channel" and yet manage to get at least 90% of their supposedly channeled Christian teaching wrong.
Today it's clear to outside observers that the WTS has never been any kind of "channel", unless one adopts the ridiculously fuzzy notion that anyone who manages to proclaim a certain amount of "Bible truth" is in some sense a "channel". But this notion is meaningless because anyone who reads the Bible out loud on a street corner could claim to be a channel. Clearly, the WTS claims far more than this for itself. In fact, it claims that its leaders have been specifically and uniquely and actively chosen by God to do a special work in the last 120 or so years.
So we have another test that the WTS fails big time.
3. The Bible and Science
We'll examine just one aspect of bogus WTS teaching about science. The number of wrong teachings is far too big to deal with in less than a book-length treatise.
Since its beginning the WTS has taught that the creative days of Genesis are exactly 7,000 years long, so that life has been on the earth for less than 35,000 years. However, scientists have known since at least as far back as the early 1800s that life has been around much longer. How much longer, they have not known with precision until the 1940s when radioactive dating methods came into operation. Today it's known that life has been around for at least three billion years.
The WTS still officially teaches that life has been around for only some 35,000 years. Why is this teaching still official? Because the last explicit statements on the subject appearing in WTS literature reaffirmed it.
Oddly enough, those who determine WTS doctrine have abandoned this teaching for internal purposes. That's why the teaching about 7,000 year creative days has seen no affirmation in WTS literature since 1987. Instead the writers make statements about the creative days lasting "millennia". However, when individual Jehovah's Witnesses are questioned about what "millennia" means, they almost always go back to the latest specific statements from 1987, and decide that "millennia" means "7,000 years". Discussions on the witnesses.net forum, in the now-defunct "Bible Research" forum, prove that this is what most JWs do.
Most JWs who know anything about science are well aware that the findings of modern science on the age of the earth and life upon it are extremely well founded. Most of the WTS's writers also accept these findings, but for "political expediency" have failed to convey their acceptance to the JW community. Really, this is sheer cowardice on their part.
We now have a third test that the WTS fails miserably. Russell began this teaching about 1880, and so we now have some 119 years of false teaching. If we measure from 1950, when radioactive dating methods thoroughly trashed any basis for a 35,000 year time frame for the history of life, we have almost five decades.
So, Friend, we have three pretty solid test cases that the Society fails -- according to your own criteria. I could go on, bringing up the blood issue and several others, but this should be sufficient to prove that the Watchtower Society is indeed a "false prophet" in exactly the same sense that it uses to condemn other religions for teaching false things.
The fact that all these religions teaching all manner of false things exist might mean that God is not concerned about it. It might also mean that God doesn't exist. Take your pick. If you say that God exists, then you must also admit that, using your own time-related criteria, I've presented three "scenarios whereby there is no basis for belief that God would overlook false prophesying".