Damn, my post disappeared -_-. W/e I'll just cut to the points.
1/It's more a testimony of her own large reserves than the effectiveness/quality of the bone warriors. Yeah, she can summon a lot of them but they don't seem to cost all that much energy. She can still put them to good use as decoys though. I'm sure she could, with time, raise a legion of them.
2/No, it's stated somewhere in CM. I'll look for it. Here;
Fate/complete material III: World material - FAQ with Nasu: Noble Phantasms, p.134
Q: If Caster had a skill (dragon summoning skill) that let her use the Golden Fleece, would she be able to escape the rank of being the weakest Servant? Also, will the spirit in the Golden Fleece ever see the light of day? He's cute so I wanted to see him in color.
A: She'd move up to the middle rank, I think. Oh, but, the Colchis dragon isn't that strong right.
Still, it's a Phantasmal Beast. I do think she's good without it anyway.
3/Nah, Fuyuki became her territory, which means she can deploy it on a large area in a short time. I don't know how big the Nexus is but it should still be bigger than Shirou's town.
4/Oh I'm not discussing her qualifications nor how much efforts it would require. Simply, it's not something as easy to use. (And again preparations are needed)
Then, the means to bring PCs to enter it, is up to you.
5/And that's fine. It's only me raising the possibility.