I can recall the Utd - Real CL game where Nani was sent off, in that game Rafael handled on the goal line which stopped a goal but no penalty was given as it was deemed accidental. I'd be all for any handball, whether intentional or not, that gains a clear advantage to be penalised but as far as I'm aware that's not the rules.
I don't remember that to be honest but from a search I found a video of him stopping a goal vs RM and the video says "Rafael red card for handball against RM" so I would be amazed if they didn't get a penalty if the player got sent off.
The problem with what you are saying with handball is that if the rules were as you would like, players would start trying to hit arms in the box for an easy penalty.
Unless there's something specific in the rules regarding scoring with a handball, which you suggested there was, then by the letter of the law Solanke's goal should have stood. Solanke neither moves his arm to the ball nor has he intentionally put it in a position to make it likely that the ball will hit it - a cross flicked off the defenders boot, onto his chest and onto his arm. It was completely accidental.
I don't think there is anything specifically about it but its a very simple cause and effect decision for a referee. Did it hit his hand then go in. If yes that is a clear and massive game changing event and should be stopped. Hitting someones arm in the box elsewhere is usually very much up for debate as to what would have happened if the arm wasn't hit.