'Endgame' effectively assumes both 100%+ eidolon and a selection of SSRs at the same time. So effectively, your starting roll will not set you up for eternity. That said, I think that for those rerolling, a 100%+ eidolon is strongly preferred over at least two good SSRs of the same element. I'll explain my reasoning for this position (which may or may not necessarily match others' logic). This won't directly answer your questions, but I hope that seeing the reasoning would let you infer what my answers would've been anyway. Except for that last question. My answer to that is, 100% eidolon + a highly desired SSR or two would be ideal, but that comes with quite a bit more expected rerolling time compared to shooting for just a 100% eidolon. I don't know how much tolerance you expect to have for rerolling :/

Short version: Power gap, probability, and power creep.

Longer version:
1. Power gap - the difference in impact between a 100%+ eidolon and event eidolons is both more straightforward and larger than the difference in impact between a typical* SSR and a good SR. Also, possession of a 100%+ eidolon makes it a lot easier to add friends with 100%+ eidolons of their own. That is, if you don't have a 100% eidolon, you don't expect someone with a 100% to accept your request. So you're effectively dependent on random non-friends showing up as well as people already on your friends list suddenly rolling a 100% eidolon and staying friends with you. So part of the time, the gap further becomes the difference between double 100%+ eidolons and 1xEvent/1x100% or 2xEvent.
*of course, not all SSR are created equal. Some can be rather batshit broken good.

2. Probability - it's hard to roll a 100%+ eidolon. If you happen to roll an SSR, and that SSR ends up being an eidolon, odds are that it'll be something that sucks until a few limit breaks later. On the other hand, if that SSR ends up being a kamihime, it'll usually be some sort of upgrade to varying degree over to an SR of that element. That is, it's easier to eventually roll some random collection of SSR kamihime than it is to roll a 100%+ eidolon.

3. Power creep - the only things that will beat a 100%+ eidolon are next year's 100%+ eidolons. That is, the current set we see now are 100% at 0* and go up by 5% with each limit break (with Managarmr and Anubis being dependent on the number of sub eidolons matching their element). Next year, we'll start seeing the release of similar for the other 4 elements. Hanuman (wind)/Nidhoggr (thunder)/Leviathan (water)/Cerberus (fire) also start at +100% element at 0*, but they go up to 120% based on your sub eidolons. Each limit break raises the floor and ceiling by 5% (correct me if I'm wrong here). So if none of your sub eidolons match element, then they ought to provide the same bonus as the old set of 100%+ eidolons with the same number of limit breaks. But they become better the more you match (4% per matching sub eidolon).
So there is power creep on the horizon even for eidolons. That said, the old set of 100% eidolons will still be plenty strong, especially if you don't like having your selection of sub eidolons restricted to a particular element.
For kamihime on the other hand? Power creep is real and constant, as expected from these sort of games. Got to get people to constantly spend, right? And the consistent side of power creep is expressed through kamihime design. What's core now may not necessarily remain core later.