Well the AI is hard that is a fact.
I play COH for a while now and even i get my a$$ kicked by expert, specially whermacht.
on hard and expert they get faster resource and mp so can pump units out much faster.
It is better to play against human players.
And like being said before you need to know the mechanics of the game plus how the factions play.
1)The AI is only hard if you give it time, its real weakness is that it thinks mechanically.
2)Expert computers need to be pressured throughout the entire game, let them get too many points and you'll be a bridge too far!

3)Hell no, especially if you're in my shoes, I keep my skills sharp by using AI, if I played against friends, my skills deteriorate! I remember once when I was vs. a friend, (in beach assault 2p) that I completely sorrounded his base with MG emplacements (yep, the entire corner) built a tank trap near the middle of the base and had an engineer flame it for ten minutes before I realised what i was doing

4) YES, you're correct but the brits are probably more defensive because of the AI rather than doctrines...
(WHO SAID CREEPING BARRAGE WAS DEFENSIVE?)