I honestly think this is the most likely explanation for this horrific event.
Actually I don't think this was an arranged professional hit at all, the murderer did not behave remotely like a professional hitman. Occam's razor suggests he was not. His weird behaviour including the bizarre envelope story suggest that this is a nutter who carried a gun for some reason known to himself, got pissed off with AW because AW was embroiled in a rather passive aggressive row with the hotel over the drinking area--which was an issue discussed by people drinking there. AW was trying to stop their fun, or whatever else was going on there (maybe low level drug dealing, who knows really), this nutcase decided to go and confront him but was so left field about it that AW didn't get what was going on.
I believe VW that she didn't know what was happening and neither did AW because the baseball cap chap was just weird. He probably didn't intend on killing him when he went there with his wacky envelope business but, well, he was a nutter.
Yes, he got away with it but that doesn't mean he's a master criminal, it's possible things got covered up because, well, AW Is dead, why stir the pot.
The envelope shenanigans and the fact that the gunman let AW back into the house and then AW got shot when he went back to see what was going on all point not to a master criminal who was "in control" but a psychiatrically disturbed person, who saw AW as a threat because AW wanted to shut down the drinking area in the pub he went to. Or stayed at. Or dealt at. Trying to make sense of that level of mental disturbance isn't possible which is why no one has been able to without weaving very complex theories involving AW having a secret life with the IRA or dodgy football clubs or...