Most of these issues relate to the batch processing that occurs at certain times a month. Not sure that anything can be done about it without completely bogging the game down.
I have had coalitions declare on me, but it has been pretty rare of late. It certainly seems to be searching for a moment of weakness. They backstab if I'm fighting elsewhere or zero'ed out on manpower.
Beyond that, I think some of this might relate to the HRE in/out issue. I'm playing as Popeman right now, Popeman is in the HRE and allied to Austria. The coalition that keeps forming and then dissolving includes France and most of the south of the HRE save Switzerland and Austria (my allies) -- and I definitely deserve to be crushed by the coalition. It would be about 200k troops against 100k troops.
I think that because France is outside the HRE and I'm in the HRE, if the coalition fires and because Austria would honor the call, it calculates whether to DOW by taking into account all of Austria's allies too, just as if France attacked me outside of the context of a coalition.