Dear Mathias!
Thanks a lot for the rapid answer!
Regarding the icons - yes, I can confirm, most of the icons I use have become suitable for FHD resolution. Only "Up" and "Eject" are old a little, causing they haven't enough pixels and don't fit good to the common modern display. We'll be waiting for the re-painting it in further updates
(check first prtscr please)
So why is DropDown Menu Font size 14px, isn't it too much? What was this value in previous 9.x version? All the other fonts are adjusted to 8px.
I wouldn't say I'm wanting to run MC as 100%. I would say, I wish to run it like MC 9.x
Please, check the second screenshot. Here are V10.1, V9.8XP, Win Explorer. They are just running all together to be able to compare the scaling.
So what I would say - here is a perfect fonts consistent in V9.8, and here is an evidence little issue with the DropDown Menu font size in V10.1. (And also the Command LIne Prompt font is larger than it was a little).
So I (and some other users I suppose) will really appreciate if You could manage fonts will be appeared as before, as in V9.8! 
Also, I tried to run MC V10.1 with different DPI settings in Porperties, as You said. No matter, scaling and fonts appearing is always the same.
I didn't know that Beta testing is open for everybody, I could use it, if You'll suppose some of my reports could be useful.
Be awaiting for further fixes in further version! Thank You, Mathias!
Yes,things are changing between version. The app is not static and never changes. well 9.8 XP edition almost is.
But as you see the UI is little smaller in total for 10 (toolbars, all all of that ).. so more pixel for the filelist in v10
Some icons does not exits in all sizes. all icons are free use icons I found somewhere. I can't draw my self. So the icon will changes if other free icons are found or
if somebody with drawing skill can contribute it.
Don't mix up 8pt font and px. 8point is not the same as 8pixels. What it was before I don't know maybe 9pt or 10pt but it does not matter. The entire combobox is new and not the standard one. it works different and support more features. I would say the font was to small before dependent on the size on the combobox. That was more noticeable on 4k screen with 170-250% scaling.
Problem is also that it is impossible to be perfect in all scaling mode because of rounding errors. Fixing for on size will create issue in another size.
Problem is that GDI UI is not made to be scaled from the beginning. the DPI scaling is a workaround MS added to Windows when the need come. With modern UI it is handled almost automatically. (Like WPF, UWP, WinUI) But for now the app is stuck in legacy UI framework.
9.8 has major issue in higher dpi settings and the standard combobox does not support dark mode, so it will not be changed to what it was.