if you have multiple VCs at each site then yes you can offload and build a standalone SSO server (I must of skipped that part) and have all VCs point to it. This will still need to be multisite to work with Linked Mode.
if VC goes down, it won't affect other VCs as SSO will be separate
If SSO goes down all local VCs will be affected with authentications but VMs continue to run
if site goes down, SRM will run a failover and use the local DR SSO instance (multisite), there is no site failover with SSO Multisite