The rest are more Symbian OS related and would better serve others if posted on the Symbian OS side of the Knowledge Network. Anyway, here's the answers:
3) True - there is no differentiation between MMSC manufacturers,
4) Already you can insert any MIME type object with the MMS Client. The client tries to recognise the MIME type but if it doesn't you can tell the type with CMmsClientMtm::SetAttachmentTypeL().
To be able to receive a new MIME type you have to create and install a new MIME type Recognizer. (More information about these in the Symbian OS Discussion)