@Matthev -

There is NO reason to run setup as the OM Server action account. This is actually a worst practice - because the user installing SCOM needs WAY more rights and privileges than the OMAA, which is a service account and should follow the path of least privileges needed. I discourage that practice. Service accounts should be granted ONLY the rights they require to effectively execute the services and processes they run. The best practice is to run setup as your user account, that has been granted rights to all the machines and SQL. Then - if needed, your rights can be revoked, and the service accounts are unaffected.

How SQL is installed is up to the organization. Generally speaking, whomever installs SQL server is completely irrelevant. Once it is installed, the security access list can be configured within SQL. Some organizations use the built in local accounts to run SQL servers, and some use Domain accounts for SQL services. Some organizations run all SQL servers under a shared domain account, and some create a unique domain account for each and every SQL server. The only bad practice would be using a single domain account, because if that account is ever locked out, you risk shutting down a LOT of SQL servers.