where is the vCenter DB located? is it on the same machine where vCenter is installed?
If so the clone will help. You can safely power down 5.1 and powerup 5.0. the hosts will show disconnected in vCenter inventory. A reconnect will solve the problem by reinstalling the agents on each hosts.
I would recommend to go with the first option ie, Try to keep f'ing with a 80% working vcenter 5.1.
Whats the error you are getting when you run VUM? post the details of the VUM and SRM problems. Hope the experts in community can help you to get it resolved.