1. Considering that various books throughout the SR history has hinted at / mentioned downcycle Dragon Hunting.. as in, some of the Immortal Elves killing off sleeping dragons, it can probably be assumed that the IEs did not hybernate through the 5th world.
3. I don't know if they regressed or not, or if there is even much information. Apparently Ehran the Scribe did a lot of Politicking in a human guise, but that could have been regression, or just a bit of makeup, disguise, or retained magic at work..
5. Similar to 3, I don't know if there is any actual information, but the likely scenario (based on UGE) is that as magic reduced, Metahuman babies just started getting born as Humans.
8. Probably the only one I can answer partly for certain. Runner's Companion (SR4) Had rules for creating Metavariants, and Sapient Species as characters.