1: Are you using the compositing mode in shadow-catcher? If so, it is slower for some reason, i guess you will have to wait for the devs to fix this/ develop it more, seems like the black background is taking a lot of the resources. :D I have no clue.
maybe try one of the other modes, might be faster.
2: I think the solution to this is to render everything with passes, comp it together with masks, and change the reflection intensity in post. That way you will have a lot more control. U can also render only the reflection pass of the ground , as one separate pass alone.
I think u can also use the AO with directionality to get the shadows.