Thunderbird will display SJIS emails just fine. The problem with attachments is when some adds a ZIP with SJIS filenames, but then it's not Thunderbird's problem but whatever tool you use to decompress it.

Regarding Python, the default behaviour when decoding and invalid UTF-8 strings is to raise an exception. But your comment made me research it and I just found that there is a way to replace invalid bytes with U+FFFD, so I will try it.