Yeah, definitely get off the aircraft ASAP and proceed directly to immigration. That's going to be hard if your mobility assistance needs are "to seat" but if your needs are "to gate", then leave with the F pax without hesitation, and get to the mobility assistance on the jetbridge. Assistance people should take the passenger through the crew/diplo/etc line, not wait in a long queue, but you still want to get to the actual immigration desk ASAP.

I might guess 2 hours would be OK, especially if there is an acceptable backup plan like later flights that are likely to have availability. If the hotel can be cancelled without penalty, or you're prepared to eat the cost, that is also a handy Plan C.