Ooo, deep questions. I have to say for me, you are touching upon some of the weakest parts in the worldbuilding of WoT. Jordan hints at things, but I think that they don't really add up if you look at them too closely. I'm sure that many others feel differently, however. I'll try to give my answer your questions briefly.
1. Yes. Each Age appears to have major events that start and end them. We don't full details of what these are, or any real details how they differ between turnings. But we can see that the Seventh Age is ended by something truly apocalyptic, so that the First Age, our Age, starts from the literal Stone Age. We could guess that the Second Age starts with the discovery of the One Power, but it is opaque what would end the First Age. The Second Age ends with the Bore being made and the Breaking. The Third Age, Rand's, ends with Tar'mon Gaiden, where the Seals on the Bore fail and the Dark One has the opportunity to escape from his prison. These appear to be constants in turning of the Wheel, though as I say, what variation is possible is never explained.
2. If the Light wins, the Bore is sealed and the Dark One can no longer touch the world. If the Shadow wins, the prison is broken open (to what degree is not clear - possibly only that the Bore is maintained, or enlarged without actually freeing the DO) and the Dark One reforms the world in his own image in the Fourth Age.
3. and 4. Where it gets tricky. Ishamael seems to think that this special turning of the Wheel it is possible not just to free the Dark One but to break the Wheel completely and end time. Is this an unreliable narrator? Is it a way of upping the stakes and fit the story to a linear narrative? One take is that only if the Dragon submits will the Dark One truly win and get free, which has never happened nor ever will (as we see in Rand's flicker-verse lives), until it does when it will happen everywhere at once - as the DO being imprisoned is a constant throughout the flicker-verse, or him being free. That is why killing Rand straight off might win the Last Battle, but not get the DO what he wants. It is important to remember that the Shadow have foretelling and prophecy as well, and this could be pushing everyone to set up the same situation where the victory conditions are possible for both sides. If the Dragon doesn't make it to the Last Battle, then the Bore cannot be repaired and nor can the prison be destroyed. Possibly the Third Age is the only time the Dragon - as the Creator's Champion - can submit. But then is this Third Age not the same as all the others? And what happens if Ishamael balefires Rand? Or would the Pattern not allow this to happen? What else would be in effect physically impossible?
5. Ishamael seems better at philosophy than Rand, which is not surprising. But as noted above, winning the Last Battle is not breaking the Wheel - and it is also worth pointing out that no one in the Shadow presumably actually wants this total victory, apart from the DO and Ishamael. They want material rewards in the world that Ishamael is trying to destroy.
So it is tricky - both the reader and the characters cannot be left to think - it is all cyclical so it doesn't matter if we fail. But there is not really an answer to Ishamael's argument that if it is all pain and suffering for eternity, what is the point? If the whole thing is balance, i.e. the brighter the Light shines, the deeper the Shadow - again what is the point? If Lews Therin had managed to seal the Bore completely, something else would just have had to happen to cause the Breaking and then the approach of the Last Battle in the following Age. If Rand fails completely, seven Ages later he gets another chance, where it again doesn't matter, long-term. Any improvement will directly lead to a worsening, caused by the Pattern. If someone chooses not to be a Darkfriend through free will, the Pattern will force someone else to be to achieve the same goals.
For me, if you examine it too closely, Jordan never reconciles the cyclical/linear problems, never explains why you need to fight for the Light if that will only strengthen the Shadow. I think you just have to leave these as mysteries that have an answer but we don't have enough information to see.
I hope none of this crosses to deeply into spoiler territory, I have tried to only touch upon the issues that are brought up and illuminated in the first four books. My sincere apologies if I have overstepped the mark anywhere.