1) I don’t have an MVS but there is a known bug with current firmware that affects some sources. Do you have a suitable SCART to VGA cable? (something like a Sync Strike) If so could you try attaching it to the AV3 input and seeing what happens? The other possibility is the MVS is just outputting a refresh rate your HDTV doesn’t like. OSSC doesn’t have a frame buffer so there’s nothing that can be done about that unfortunately.
2) Never seen this, maybe you can post a picture?
3) Remember you should display Line triple in 16:9 mode on your TV, even if you choose generic 4:3 line triple mode.