Thanks so much for such a thorough, helpful response!
The Worbla/Wonderflex idea is an interesting one, but I could see that getting pretty pricey, and I wouldn't want to completely reconstruct the tail. Unless you meant just putting the support in the base, which would be doable - it's sewn together pretty shoddily (it's hard sewing fur!), so I could always rip that apart and shove something in there. I'll have to think on this some more! I guess my biggest concern would be making sure the support isn't noticeable underneath the leotard (and I'm not exactly skinny, so I wouldn't want it add more bulk).
I would prefer the tail to be removable, on the off chance I need to drive or use the bathroom or something, so I wouldn't want to sew it on (and honestly, I'm not sure how easily it would be to sew it on, since I had so much trouble just sewing it shut). I'm okay with having the top the way it is now, because as long as the bottom detaches easily, I can just swing it sideways to sit - though making it completely
detachable (and easily attachable) would certainly be nice.
I had actually left the crotch unsewn and added snaps, but the placement was bad and since I was short on time, I just ended up removing them and sewing it shut. I'll probably try to add them back in at some point (or a zipper), because if I need to use the bathroom... the entire thing has to come off, haha. Fortunately I did think ahead and bought a separate top and leggings instead of a full unitard. As for the opening, right now it's in the back (the hole the harness strap goes through is right under the zipper end) and I don't know if I'd want to move it to the side instead. I could see that making it easier to put on, but I don't really trust my sewing skills (i.e. I don't want to accidentally ruin the unitard and have to remake it.. for a third time).
I do really like your button idea. I'd worry that a circle button might just slip out, but if I got one that was more oval shaped, or even rectangular... hmm... that might possibly work!