Richard... in what way is the opinion expressing US preference about the release either 'interfering' OR 'insulting? I can't see that anyone expressed 'entitlement' to review Scottish decisions.

"a far preferable alternative to prisoner transfer" is merely a position and opinion. Why would the US NOT at least clarify its attitude?
What did happen is as was feared... releasing him to Libya served to give him hero status and further degrade relations with the US.