Thank you for your input.
Yes, all NICs run at 1 Gbps (as can be seen on the screenshots). The RSS queue value 2 of the various problematic NICs doesn't seem to be a problem for Windows' SMB: For testing purposes I reduced the properly working I350-T2's RSS queue from 4 to 2 and the PowerShell commands mentioned in posting #7 still attest RSS being active in Windows.