IMPORT FOREIGN SCHEMA fails with Password error for 127.0.0.1 but not for remote IP (of same server)
The problem is that when you connect locally, PostgreSQL does not demand a password. So while the user mapping might define a password, that password can't be used as it is never asked for.  If it doesn't ask for a password but can still connect, that probably means it is relying on the identity of the process doing the connecting to authenticate it.  Since the process doing the connecting is owned by a service account which is running PostgreSQL (usually 'postgres'), you are leveraging an identity you don't have a right to leverage. For security reasons, this must be disallowed.
Since you are using a hosted service, you are at the mercy of the host to fix it.  There are several ways they could fix this.  There are no ways you can fix it without their cooperation, other than to stop using their service.

&quot;Allow only SSL connections&quot; using the remote IP fails because I don't have certs

SSL certs are generally going to have the host name, not the host IP address.  Did you try with the host name?  It might be that Google automatically installs their own certs (or that you can talk them into doing so)