What should/can I do if I'm waitlisted?

I hope I'm posing this question in the appropriate sub-forum.

I applied (R1) to a top 10 school and was recently waitlisted WITHOUT interview (weird - I expected outright rejection given the lack of an interview). I read their waitlist FAQ and it stated waitlist candidate will be informed as soon as April/May or even as late as August.

So my first question is, what (if anything) can/should I do to improve my chances as a waitlisted candidate? The FAQ says I am given one opportunity to "update" the school about any "significant personal changes"; is that a hint that this is my shot to improve my candidacy (IF I could do so within the parameters of a "significant change"?)?

Second, realistically speaking, what are my chances if I wasn't even invited for an interview?

Lastly, during R1, I was also accepted to a top-30 institution. The deadline to accept/decline their offer is Mar. 15th. Yet I probably won't hear anything from my waitlist school until at least April. The safe thing to do would be to accept the admissions offer (& pay my deposit). But my question is, if by chance I actually get accepted to my waitlist school, would I incur any serious negative consequences (legal, reputation, or otherwise) if I accepted the admissions offer but attended my waitlist school instead?