(I’m not a dev. I’m not evaluating your bug report. I’m just replying to a few points.)
You linked to the help docs that explain this. Obsidian wiki links and Markdown links are not handled the same way. Markdown links need to be URL encoded.
If you Ctrl-drag, you’ll get an absolute path to the image on your filesystem. If you drag, yes you get a relative path in the vault. That’s how Obsidian works. Relative filesystem paths is an open feature request, iirc.
I don’t know about every other point you made about compatibility. But here is a statement that WhiteNoise provided about compatibility issues. In particular, wikilinks are addition above Commonmark:
And if you open a feature request, please search the forums first to make sure it doesn’t already exist.