@ York Zhang
1. On the amount of time to wait, I'd recommend giving it a day to let things stabilize, however if you need to perform both steps inside a maintenance window, then I'd say give it at least an hour just to be safe to ensure that config catches up in the management group, and check to maker sure everything is working properly before proceeding.
2. Yes - you can leave all your agents alone, and reject the pending update after applying SP1 to the management group, then only update them a dingle time with SP1+UR2.