There's no rabbit hole of logic in the case law and precedents. If someone is legally carrying a firearm, then the crowd has no right to disarm him and the attempt to disarm him constitutes an attempt by them to apply deadly force. The other 4 facets need to apply as well. If he is acting threatening to them, or had at any time in the past to them, then they may be justified taking his firearm. But, just because you hear a gunshot and then later see someone with a rifle slung over his back does not give you the right to use force on that person. It's situational though, if he was pointing his gun at people and doing anything other than retreating and saying don't come any closer or I'll defend myself, then you might be within the law relieving him of his weapon. The thing is, normally in that situation, either you or someone else needs to be in imminent threat of bodily harm or death, so you need to be able to articulate who was in that state. You cannot use force just because you don't like guns and someone else has one (and of course if it's a little kid or someone who obviously shouldn't be possessing, then you're probably ok there as well).
Rittenhouse would be different as he would've heard the gunshot as he was already being chased and was attempting to flee. Pretty much any reasonable person would connect those 2 in the crucible of being chased. More than that, the fact that he had a rifle helps him because the mob would be able to see his rifle and still they came after him. That would play a factor in anyone's state of mind.
As to the next step, if after Rittenhouse fired someone from the mob that had been chasing and threatening him had then shot him, likely they would've been taken into custody and later charged with murder. Because the first of the 5 facets of self defense is innocence and you cannot claim innocence if you are part of a mob chasing someone down threateningly.
Rittenhouse, who was directed by police to go back where he came from when the first mob recognized him as someone who prevented them from destroying a car dealership attacked him, can likely claim innocence because there's no evidence that he was actually threatening anyone at any time prior to that. At least that I have seen. the fact that Rittenhouse even stopped to try to provide care for the first wounded attacker and that later he tried to turn himself into police will help Rittenhouse. The second mob pressed an attack while he was trying to provide care to the first victim when he fleed, again not posing a definable threat to any of them. They struck him multiple times and still he didn't fire.
IMO, the prosecutor is going to have a hard time disproving any of the facets of self defense (innocence, imminence, avoidability, reasonableness, and proportionality)