Why did Regret attack Earth then suddenly change his mind and find Halo?
Regret went to Halo because while oh Earth they discovered the coordinates for it. Halo is obviously more important to the Covenant than Earth.
How did the flood get on High Charity and how did the Gravemind get on if he was under miles of rock?
The Flood got to high charity via Commander Keyes’ ship In Amber Clad. Gravemind could have easily been rebuild on High Charity using newly harvested biomass, or used Halo’s teleportation grid on himself, as he did for Master Chief.
If activating the ring made Truth a God, why did he let Tartarus do it, wouldn’t he need to do it himself?
The Covenant belief is that activating the ring will make ALL who truly believe and follow the path into a god, not the activator.
Why did Truth go to find the Ark if he would have already been made a God from activating the ring, and how would he know about it in the first place?
Truth learned about the Ark during his interrogation of 343 Guilty Spark. He went specifically to investigate, and partially out of fear as the Flood, and Demon were both still a threat at Delta Halo.
Is the forerunner ship from High Charity the same ship as the one in Halo 3?
Yes, the Dreadnaught is the exact same ship.
How did the flood get on the heretic station and why did they suddenly break out of their prison?
The Heretic Station is a Forerunner flood-study outpost. The Flood were kept there and analyzed constantly by the sentinels to be more understood. The safety was the cable, which would plunge the station into Threshold’s gravitational core preventing any Flood escape, as it did.