How do Jehovah’s Witness support their belief that Jesus started ruling (invisibly) from heaven in 1914? What has been happening in those 107 years?

Obviously, in any set of beliefs, certain matters overlap. Some of the quotes you gave relate to the belief that 1914 was the beginning of Jesus's rule in the heavens, but are not necessarily a fundamental part of it.

1. The first important point underpinning this belief is the understanding that Jesus 'coming' is a presence. You can read an official explanation of that [here] 1 A fundamental part of this is that Jesus said that the manner of his 'coming' would be in the same manner as his departure (Acts 1:11). While some denominations have focused on the fact that his departure was miraculous, ascending into the clouds, and imagining that his return would be some big, miraculous event seen in the clouds, Jehovah's Witnesses focus on the fact that he departed quietly with only a few faithful disciples watching, supporting the belief in an invisible presence that only they recognised.

2. Your first official quotation contains details of how 1914 is calculated using biblical dates and prophecies. An important detail to know is that Jehovah's Witnesses arrived at this calculated date long before that year, but they did not always fully appreciate its significance, and the date may have been associated with other biblical prophecies. Their belief in it being a pivotal date has never changed, even if their understanding of what it meant has been adjusted.

3. Supporting the above belief, Jehovah's Witnesses relate all other Biblical prophecies concerning "the last days" to that date, for example the signs of the times in Matthew 24 that include wars and earthquakes, the attitudes of people described at 2 Timothy 3, as well as Revelation prophecies including the 4 hors*m*n. They see the prevalence of these events and attitudes, or an increase in them since 1914, as evidence that the 'last days' of the present world system of things began in that year.

4. The quote you gave about a shift in belief regarding 'a generation' is related to the last days (the time period beginning in 1914) but not directly related to the calculation of 1914 itself. This belief refers to Jesus words at Matthew 24:34 which state that the 'generation' witnessing all these signs would also see the end of the last days. Their current understanding is explained in [this article] 2, but simply put, their initial belief (until about 1995) was that a 'generation' was a human lifetime, so therefore they believed that the 'last days' and the end of this system must come within the lifetime of anyone alive in 1914. Around 1995 this was adjusted with the understanding that a 'generation' could be an indeterminate period of time rather like "an era" or "an age". But in recent years this understanding was adjusted again, and the current belief outlined in that article is that the generation is the overlapping lifetime of two sets of contemporaries. In some ways, this shortens the possible length of the last days as it is once again restricted to human lifespans. Throughout all of these adjustments, the belief that 1914 was the beginning of this period has not changed, and Jehovah's Witnesses have always maintained that the last days would be a relatively short length of time and must end 'soon'.

5. The quote you have about an uncertainty surrounding the year 1914 may be out of context as it is from a source that is negative about Jehovah's Witnesses. I can't see that there has ever been any uncertainty about the year, only about what it might mean and how long the period after it may be.

6. They believe that during the time since 1914 on earth the world conditions described in the prophecies mentioned above have worsened - that prevalence of wars, earthquakes, disease and ungodliness has increased. They [link this] 3 to Matthew 24:8 where Jesus likens the signs of the last days to the labour pains of a pregnant woman and understand this means that the signs will intensify and become more frequent. They also believe that their work of "preaching the good news" is helping to fulfil the body of prophecies at Matthew 24. Verse 14 prophecies that it will be preached to "the entire earth", and they believe that their international organisation is the only one doing that in a unified fashion.

7. As to what has been going on in heaven since 1914, Jehovah's Witnesses believe that God's Kingdom is preparing to begin ruling over the earth. They believe that the beginning of Christ's presence was also the beginning of his rule as king of the kingdom in heaven.

They believe that his first act as king was doing battle with Satan and the demons (Revelation 12:7,8) and that this resulted in them being expelled from heaven, confining them to earth (Revelation 12:9). Prior to this, they believe that Satan could come and go freely between heaven and earth (Job 2:1), and that this confinement to the earth since 1914 also directly contributes to the worsening world conditions since (Revelation 12:12).

Another thing they believe has been going on during that time is the gathering of 144,000 co-rulers, a limited number of humans that have been chosen to rule over the earth with Jesus, and are resurrected to heaven upon their death as opposed to the rest of faithful persons who hope to live forever on earth under this government (Revelation 14:1).

This number is significant, as the Bible teaches that the Jews were once God's 'chosen' people - not the only ones acceptable to him, but in a covenant arrangement with him, and these would have been the ones to rule in heaven if they had remained faithful as an entire nation (God organised the Israelites into 12 tribes, 12 being a 'complete' number in Biblical numerology. 144,000 would have been 12,000 from each of those tribes).

They believe that the majority of this number would have been chosen long prior to 1914, not just from the modern-day organisation of Jehovah's Witnesses, but from Christians in the first century and potentially sincere ones from the many centuries in between.

They believe that those chosen prior to 1914 only received their resurrection there at that time, but that the remaining ones in modern times are resurrected to heaven immediately upon their death.

Also, they believe that as Jesus is head of the Christian congregation (Ephesians 5:23) he is actively watching over true Christians on the earth, invisibly overseeing matters within congregations. They also believe that Jesus is invisibly directing their preaching work, as he is the 'master' in his own illustration recorded at Matthew 24:45, with a 'faithful slave' on earth overseeing it.