Slitherine

or there any relaxation of wheeling restrictions for congestion when multiple units assault one unit? (rules page 29)

When multiple units are assaulting one unit does "or as far as possible if this would otherwise cause them to not reach their target" mean that in order for multiple units assault the target they can each wheel less than " must wheel as to place some part AA in front of their center of their own unit"
( units 1,23 all assaulting unit A)
1111 2222 3333

AAAA

if 111 and 333 both wheel to place some part AA in front of their center of their own unit then unit 2 wont make contact (1 and 3 will in the way if 2 goes first, 1 and 3 can ONLY make contact if they dont wheel to place some part of unit A in front of their center.)

And if they can 'adjust for congestion' can they ignore the point "they may not wheel to place less of their target to their front, or past" bit if that is the only way they can make contact due to other assaulters? (or obstacles )

I would day in this situation 3 could not declare a charge as the other two units would block it, this making contact impossible so no charge, unless 3 had enough movement and space to hit a side edge of their target, even that might be impossible. We had a situation like this in one of our recent games, 3 French units wanted to hit a single Austrian unit. There was not enough room for all 3 French, so one charge was canceled and they moved up for rear support instead.

I don't believe there is an exception. Sometimes there just isn't enough room for everyone given the time and path.

Exactly, it's a matter of physical space and can you actually fit into said space. Don't be so greedy, 2 on 1 with rear support should get you your desired victory.