First, a couple preliminaries/considerations that don't directly answer your question
- Have you tried anything other than emails and gossip?
- Not trying to be harsh, just some people respond much better face-to-face.
- Are there office hours for the professor?
- Is there money involved?
- Has the funding explicitly/implicitly already paid for your tuition?
- Has the funding paid for anything else?
- Will it go away if you switch, or at least require significant behind the scenes money shifting?
- Is project A still available?
- If I was a professor, and had a student choose B, then I'd look for student for A.
- Research doesn't stand still, and often there's a limited time window.
Given these, resolve Question 1:
- if( !(Tried anything other than email) )
- then: Attempt to have a meeting with the 2nd professor anyway, even without an email response. Or trying calling them and leaving a voice message. Or go to their building and try to arrange an appointment (maybe a receptionist?). Or, show up at the office hours and introduce yourself. Talk about your concerns politely, yet honestly, especially if you feel there's a need for significant research guidance.
Resolve Question 2
- if( know money is involved )
- then: Show far more care about changing. If you do meet with your Advisor or 2nd Professor then its definitely something to discuss. It's not so much an issue of flakiness, as whether your education will suddenly stop having funding.
- elsif( don't know )
- then: Find out. If your Advisor is easier to find and talk with, then its a point that probably needs to be comprehended.
- elseif( money is not involved )
- then: Far less issue with changing research topics.
Question 3 resolution should be fairly obvious
- if( project A was assigned )
- then: You're probably out-of-luck. Especially if student on project A has already started working and performing research.
- else: Far less issue with changing research topics.
Personally, I think I'm slightly less positive on switching than the other answers (especially if you've only tried email). You're in a profession where you're expected to track down answers, and pursue (somewhat) independent lines of study. I'd expect that person to at least meet the other professor. That said, understand the issue as I had a PhD experience with very distant advisors, and many undergrads => PhD's go from lots of advice => "nothing".
If you try meeting 2nd professor (even without an email confirmation) and it still does not work, and there's limited risk for education support (money), and Project A is still available, then its probably acceptable to request a project change.
Meet with your Advisor. Get their opinion on the situation, explaining what you've tried and your concerns. In their position, I would probably pick up the phone and try calling 2nd professor. If you're set on switching, try to make the switch as painless as it can be, and offer to do whatever legwork is necessary. IE: Don't just dump it on the prof. and expect them to deal with it. Personally, I find that passive aggressive. Also, I cannot stress this enough - try more than just email. Do you want to go to your Advisor, and they'll ask "what you have tried?" and your response is "I sent some emails."