(Please ask one question per thread.)
Q1. I'm not sure how to make Foobar2k show/set ratings, but in MP3Tag if I set an MP3 file's "RATING WMP" tag via the Extended Tags dialog, then the same rating is shown by Opus. MP3Tag supports two other types of rating tags so maybe your files are using different ones to what Opus and Windows Media Player use.
Q2. Use the format how?
F1. What are the custom fields? How have they been defined? If you mean custom tags then there is no way to get them to show up in Opus at the moment, short of writing a shell extension DLL.
F2. There are a few buttons floating around the forum to do that (e.g. this one). The exact details will depend on where you want the M3U file to be created, what you want it to be called, and if you want it to automatically select all music files or just use the ones you have selected.