I'd go ahead and do the substitution for the onions, with the knowledge that the onions would be slightly less sweet. But I wouldn't mind this. I use white and yellow onions fairly interchangeably in all of my cooking (caveat, I don't make curries of any cuisine very often).
I'd probably use the crushed tomatoes, to make my life easier. But could see the fresh possibly being better texture-wise, as you'd presumably end up with more substantial pieces of tomato. That is, unless you're cooking the fresh tomatoes for a long time to break them down.