Icon sizes already scale with DPI. I am not sure what you are asking for here.
Turn on Preferences / Miscellaneous / Advanced [Cosmetic]: gloss_and_gradients if you want Opus to render things in a style suited to older versions of Windows rather than match the flat style suited to the Windows version you're on.
There shouldn't be any difference in speed. Opus 9 may have waited until the thumbnail and displayed nothing in the mean time (I can't remember that far back to say for sure) but that's not good in situations where the thumbnail takes a long time to generate.
Most of the time, thumbnails will be coming from the cache and will appear instantly. The cache will have been cleared when you upgraded, so you may just be seeing that as the actual difference, until the cache is repopulated by visiting the folders again in thumbnails mode.
We'll add it to the ideas list.
It's in the menu attached to the OK button in the Create Folder dialog. You can also hold Ctrl when you click OK or push return.
If you want it to happen all the time, edit the toolbar button and change the command to:
CreateFolder READAUTO
FLAC tags have been supported for aeons. You may need to reset your Music file type group if it dates back to Opus 9, as it probably doesn't include the .flac extension and may not have a full Info-Tip definition either.
Settings > File Types, then, under File Type Groups, right-click Music and choose Reset to Defaults.
That should also fix your alignment issue. If not, you can edit the Info-Tip in the same place.
The transparency issue with the thumbnail may be an error in the source image. See if Explorer or thumbnails mode in Opus show the same things with large enough thumbnails.
If you need to follow-up on anything, please create a new thread, as it will be impossible to keep track of so many different things in one thread. Please Ask one question per thread.