Afaik, as long as a weak cipher is enabled on both client and server, a MITM attacker can force it to be used. It involves manipulating the handshake to tell both parties the other one doesn't support any better cipher.
Eh, no. Maybe in SSLv2, but the first thing TLS encrypts is a hash of the entire handshake. Modifying the cipher list would change those hashes into something different.
Unless you have a client which will happily disable a cipher and try again when encountering an error. But if you do that, you don't deserve any security.
Unless you have a client which will happily disable a cipher and try again when encountering an error. But if you do that, you don't deserve any security.