I've tried setting the zoom to all kinds of different settings, and it may have an effect, but it's a weird one. When I get an mtext editor that is unusable because of the scale, I exit it and change the zoom, then try again, and eventually I get a usable text editor. There's no consistency to it, though; it seems to happen at all sorts of zoom settings, not just when zoomed way in or way out. It's like Autocad uses some (incorrect and/or buggy) mathematical function to figure out what size to make the text in the text editor, based on text size vs. current zoom setting and maybe other factors, so it crops up when things are in the right relative relationship to each other, but I haven't quite been able to discern any sort of pattern.
And it's possible, but I don't really think the transparency thing isn't due to a setting. Like I say, nearly all the time, the text edit window isn't transparent. It's just occasionally, and then simply moving the text editor window fixes it. This wouldn't happen if I had set something that caused my text edit windows to be displayed transparently (although this causes everything to become so unreadable, I don't know why anyone would ever WANT it to happen...)