1) This is pretty subjective. The improvements are listed on their Molly's site and GitHub so you can decide whether it's worth trusting another party.
2) Oscar is the only dev and Signal makes point releases fairly often, so I imagine this is because rebasing onto every Signal release would be an unnecessary maintenance burden.
3) You're right that Moxie wasn't very fond of third-party clients to Signal servers, but leadership has changed since then and so Signal's position may or may not have changed as well. In any case, Signal can't detect whether a user is using the official client (at least not without implementing attestation for that purpose, which has undesirable implications for them) so there's not much they can do about it. Molly also has plans to run its own server network and it doesn't violate the Signal trademark unlike some other forks, so Signal has less of a problem with Molly than other forks for the latter point at least.