I think a distinction must be made between pronunciations with mekoros, or at least widespread minhag, and those without. OH, EY, and AU (yekke) are all brought in Rishonim (Rishoynim ?) oy is not but it is widespread minhag hailing from a good chunk of Medinos Ashkenaz. Adeenoy with a cheerik is entirely out of the question, despite it being “in style” among some bochurim.
Re point two: I was once told that ???? ?? says that the mel(oo/ee)pim is halfway between a shoorook and a cheerik hence the chasidish “ih.” If anyone has the citation please let me know. I’m not really a daqdeqan, I only know the questions not the answers. 🙂
Re point 4: The Mishnah Berurah at the beginning of nesias kapayim gives guidelines as to which speech defects are problematic under which circumstances.