Developer ID Installer certificate “unusable” on another iMac

You need to import the cert via Server.app; that'll add it to the System keychain and several other locations that allow non-keychain-aware services to use it. I'm not sure why it'd be hanging, but I can think of a few things to try:

• If it's not a self-signed certificate, you probably need to import the appropriate intermediate certificate(s) by dragging them into the "Drag extra non-identity certificates here" section of the import dialog. If it's a DigiCert certificate, you can find their intermediate certs here. There's probably also a way to export this from the Windows server, but I'm not familiar enough with it.

• It's possible Server.app is getting confused by the file extension. Try renaming it from .pfx to .p12 (they both refer to the same file format, PKCS #12).

• There might also be something odd about the file contents. Since it sounds like it imports ok via Keychain Access, try importing it with that, then re-exporting it to .p12 (see the instructions you linked), then delete it from the keychain (to avoid a conflict) and re-import it with Server.app. It's possible importing and exporting with Keychain Access will clean up the file and make it work properly in Server.app as well.

servermgrd looks up the certificate it uses for port 311 via an identity preference named "com.apple.servermgrd". By default, the preference points to the self-signed "com.apple.servermgrd" identity cert. After you change the preference you can remove the old self-signed cert without causing any harm. But you must make sure that the access control list (ACL) on the private key of the certificate that you want servermgrd to use allows servermgrd access.

If servermgrd cannot access the private key, it will delete the preference and create a new self-signed identity and set a new preference to point to the newly created identity cert.