There was an original timeline where Crono and co never time traveled. The entity supposively created the gate at Leene Square to start off their adventure. Impossible for the Gurus to do that.
Again, you're stating the obvious (the comment about the original timeline) and trying to use that for an argument - what you're saying is not going to hold up in backing up your argument. The "entity" may have created the gate at Leene Square to start off the events, but what we do know is that it was based off of the pendant reacting with the telepod. That was the catalyst, although the exact method of how it happened is unknown.
He was still working on everything when you meet him the first time.
He's working on inputing his memories into the Nu when Crono and co. first meet him - that is all that is revealed. When exactly the Wings of Time was finished is not known.
Like I said, its not indictive to the Gurus.
65,000,000bc- Has to do with Lavos. Nothing with Gurus.
12,000bc- Has to do with Lavos. One of the most important events in history.
600ad- Has nothing to do with Gurus, just has something to do with Magus, and Lavos.
1000ad- Has something to do with Melchior, but lets face it. It just helps out, rather than guide. Has to do with Crono, and therefore Lavos.
1999ad- Has something to do with Lavos.
2300ad- Has something to do with Belthasar, and Lavos.
Except, you just proved my point without realizing it. 65,000,000 B.C. is when Lavos fell. The Gurus studied Lavos, and at some point they must have investigated its origins. 600 A.D. has to do with Magus, who is Janus, and Lavos. We know that the Guru of Reason knew that Lavos was summoned by Magus in 600 A.D. - whether he also knew Magus was Janus is uncertain. The fact that Lavos did something in 600 A.D. by being summoned is of significance to the Guru of Reason. The rest I'm not even going to comment on, as you've pretty much proved my point that the Gurus appear within these time periods outside of 12,000 B.C. Really, this can't be taken as just coincidence.
I think Melchior was the most truthful of all the Gurus. He didn't buy it because of what he said. Obviously, it was just as important to Marle, more so that Melchior.
The problem here is he'll say it even if Crono tries to sell it to him without Marle around. You were basing your argument upon Marle being there and her expressing its importance to her. Furthermore, Melchior initially wanted to buy the pendant even when Marle was around - it wasn't until he realized it was the pendant he had made that he decided against either buying it or persuading Marle to sell it to him. The pendant's significance to Marle is of no importance.
The pillars of light didn't lead directly to the day where Zeal was destroyed. The bucket lead directly to the Day of Lavos.
The pillar did lead to a time almost immediately before Zeal was destroyed. When Crono & co. reach Zeal for the first time, the Ocean Palace is nearly completed.
The Gurus never finished their work in Zeal. Magus summoned Lavos because he was a powerful Wizard. It was via the use of Magic that they made contact with Lavos. None of this gives any of them the power to create gates.
You're right - the Gurus didn't finish their work in Zeal, but that does not change the fact that they were working on it. It's still up for debate how Magus summoned Lavos. I will give you that he used magic, but the question still remains as to how that magic was used to summon Lavos. I don't believe your argument that the Zeal made contact with Lavos purely through magic. It's not fully known just how Lavos was discovered during that time period. There are two things you are also failing to take into account here:
1) The Wings of Time was eventually completed and was able to time-travel. The mechanism for its time travel capability is unkown, but it was based upon the Guru of Reason's research.
2) Lavos could create gates, and we have seen that the pendant also can create gates or other temporal distortions.
If he had access to the Epoch (meaning it was finished) give me a good reason why he would have still gone insane.
What you are not considering is the possibility that completing the Wings of Time was part of the plan to change the planet's future. Had the Guru of Reason not completed the Wings of Time, or had he not decided to even work on it, he still would have gone insane in 2300 A.D.