Hello @Charlie Grey ,
the bootstrap protocol supports multiple BSRs and an election process of a Designated BSR and multiple Candidate RPs so this is the way to go.
Q1: if the MA is down the multicast can still work if : it is working in sparse-dense mode or a last resort RP is manually configured on all routers ( remenber that dynamic info overrides static info in mapping RPs to groups)
Q2: Multiple mapping agents can be configured , but they miss an election process like BSR has, so they will work all together at the same time collecting messages from candidate RPs creating their own RP to groups mappings and then sending it.
And yes you can configure the mapping agent on the same router that is a candidate RP.
Hope to help
Giuseppe