Okay, guys -- re-watched "Welcome to Happy Harbour". There is NOTHING in that episode that implies that Red Tornado is not compromised by "The Light". We don't know what the "reprogramming" was that Morrow's assistant was trying to do, Maybe it was just a service pack upgrade.
We DO have this rather odd quote by RT "If I were to intervene it would not be to help". His attitude towards not helping is rather at odds with Batman's later "we expect you to ask for help if you can't handle it" policy. (as I recall without checking) "There are things we can't handle alone, that's why the league exists" Both those sentiments take on a different implication if you think RT is working for "The Light"
We also don't really know that there was no opportunity "off camera" to reprogram RT between that episode and "Infiltrator".
However an additional possibility for the "operative on the inside" does occur. Wally's "eye of Mr. Twister" souvenir -- anybody ever see "Sky High"? ("He's got an ego the size of a giant robot")