When you use a client, you are relying on the client's crypto implementation to be correct. This is only one part of it and there's a lot more to it when it comes to hardening the program. Signal focuses on their desktop and mobile clients and they hire actual security professionals and cryptographers (unlike the charlatans in this thread) to implement it correctly.
Having third party clients would not definitively mean the client is bad, but it most likely would break the security model. Just take a look at Matrix's clients.
This is entirely dependent on the situation. Privacy is not a black or white thing where you're completely private or not private at all. Everyone lives some part of their life publicly. I don't have data on this unfortunately, but typically where I live, people share phone numbers to people they personally know.
This is not within the vast majority of most peoples threat model.