In answer to those questions:
1) The way mirror mode currently works is by using the connection you've made through the legitimate client to do it's thing. This means that mirror mode will (for the foreseeable future) not receive proxy support, as it is not OSBot that establishes the connection to Runescape, and thus can't modify that connection in any way. However, there are some unofficial ways to run mirror mode via a proxy. There are a couple of tutorials to be found on the forums.
2) Yes it is, @MGI is still working on improving the client where possible.