Yeah, thats a lot of questions. Depending on the Midlet, they use specific device APIs that you can't use on a blackberry. E.g. Nokia has special UI APIs and you can't run those on a blackberry. That is one of the big misconceptions, unless the application is truely a J2ME app ( i.e. only using CLDC, and MIDP ) can it be portable across all j2me devices.
Hopefully that makes sense if not I'll try to explain further. You can get the RIM JDE from www.blackberry.com/developers
but beware this is a development environment tool for development.