First thing : it’s impossible that all issues you see are from darktable code. I follow regularly pull request and issues posted on Github, I’m on the main site and french forum about darktable and here, I never see that. Icons are normally not misaligned (I change as I’ve said alignment of iop icons. I have even made these last days new corrections and improvements from other issues I see (doesn’t include icons alignment) and this was follow by some people and corrected many things. I have also made myself for that quite a lot of tests (with a quite rainy week-end from last one, I have spend some time). And like for previous time, for interface issues, a capture screen is better.
Also, I don’t understand how you could have these “japanese-like” symbols. So, on what you describe, two possibilities I see : compiling issues on your system or MacOs issues. And for that, only developers who use MacOs could help and I’m not sure but they are one or two if I remember (except erdgadoh, don’t know who).
About 2nd preview, there’s a work in progress (a pull request) and issues about 2nd preview window on MacOs, on the github page : https://github.com/darktable-org/darktable/pull/2569
but you certainly know that as it seems issue related is from you.
What I’m sure is, on Linux at least, all issues you described doesn’t exist (at least on my OS but from what I see, no one else describe that).
I will respond to question you asked to @aurelienpierre as I also works on css so really know it great. Text font sizes you are emphasis (all), those you ask are on main value=1em, so the real size is related to default font size used (OS one for darktable.css, Roboto or next one used for elegant themes).