It is small, I did say that before, you can always make fonts larger, same applies to Android/Windows 10, Arabic fonts look smaller by default compared to English.
This is not true according to my testing, are you using the UI variants? Without it, both Sans and Naskh break in a similar way. See screenshots below for comparison between the two.
I’m not suggesting we use none-UI version for UI, that will definitely break many things!
Very subjective, I don’t see issues there at all, diacritics are small it seems, that’s about it.
You keep dodging the fact that Naskh is used by default on Android, it cannot be that bad, else you’ll have millions of unhappy Arabs 
To be honest I don’t see any bugs, calling it buggy is unfair!
To be honest on a personal level I’m not bothered, I’ve always used my own custom .conf file, but I really care about choosing a good default for your average user, Ubuntu is very influential.
Again, it’s up to you, not fussed, no hard feelings, anything proposed is miles better than DejaVu.
But my opinion stands still, I still think overall Naskh is well balanced font, Sans is kind of wide which gives it that feeling of “more readable” but has it’s issue, like everyone above identified.