2) The FireSight MC can be installed in a different network. Yiu just need IP-reachability on the relevant Mgmt-ports.
1) With two ASAs in HA, managing FP with the onboard solution is not an option. As there is no sync between the modules, you had to make sure that all changes are done on both ASAs. IMO this can only be ensured by pushing one policy to both modules. Thats the way the external Firesight does it.
The 2-node virtual MC has a list-price of $600, so compared to the ASA (and the licenses) it's quite "cheap".
Still, the base-ASAs are still managed by ASDM/CLI/CSM while FireSIGHT only cares about the FirePOWER portion.