Slitherine

I believe you can shoot at a large unit in contact if its only in contact with one base. This doesn't seem to be at a negative POA?
We had an odd siutation where a large cav unit in contact was shot at from close range by enemy artillery and infantry during the firing phase (not defensive fire) andd took 7 hits. This meant the cav had to retire out of contact before they had even fought. This seems to be the right way to play it according to the rules but it seemed odd that they never fought. Did we do this right?

As near as I can tell looks like you got it right. My guess was that sequence aside the assaulting cav was shot to pieces on the way in and most likely never got close to their target.
Odd though that unlike other fog rules no negative poa for shooting into a melee. Lesson here is only attack units that don't have friends nearby it, and assault with large units 2 wide so they can't get shot up before they can cause chaos.

Well that was the other unusual situation. The cav had intercept charged the infantry they were in contact with, so neither those inf not the other enemy within 2MU could fire at them as defensive fire (can only defensive fire against assaulting units, not interceptors). We were surprised at this but that seemed the right way to play it as per the rules. What do you think?

Putting turn sequence aside again what appears to have happened is this, the infantry make a grand assault. Enemy cavalry suddenly appears charging towards them. The infantry suddenly stop short yelling profanities and expecting to die. The cav looking at an easy kill suddenly finds themselves in a zone of death from canister and musket balls, my guess is they start the profanities now, and they are driven off just short of their target. Possibly some of the front cav got a few slashes in before they had to retire.

So that covers the situation would you not agree? Bad day for the cav in the end.

Yes I could picture that in reality but I wanted to make sure we'd played the rules properly.

Looks like you did indeed get it right. Does make for a weird outcome though. Personally I would rather see units in contact can't be shot at period, but they decided to pick on the large units again. Of course if your large unit was 2 wide this would never have happened.