CA A revokes CA B, does certificate C become invalid?
Yes, revocation cascades down to the tree. If CA certificate is revoked, all certificates below (regardless of how many levels are below CA) are implicitly considered untrusted. Keep in mind that they become *untrusted*, not revoked.
CA A gets revoked (somehow), does its revocation cascade all the way ...
The most secure answer is "yes, it revokes the subtree", because once the "B" certificate has been revoked there's no reason to trust any certificate it claims to have issued (or any CRL it has signed, et cetera).
But it really depends on what inputs are given to the chain builder (which means it won't be consistent from application to ...
TL;DR: It's not permitted; it's not technically impossible.
Assuming you're not talking about a certificate that's been held, it would be against the rules for a certificate, once revoked, to be unrevoked.
If you ask a CA to unrevoke a revoked certificate, they should refuse, and they'll probably tell you it's impossible.
However, if you websearch, you ...