I played through on Normal with extensive air war savescumming and moderate ground war savescumming. Point by point:
1) I agree that reaction fire on door opening makes a ton of sense and I endorse its addition. I'm unsure about your difficulty scaling. Is there an existing reaction fire scaling factor on various difficulties for other tasks? If not, this could be a strange asymmetry.
2) No strong opinion.
3) Makes sense. Unsure on specific numbers.
4) Disabling autoclosing on the big door would be fantastic.