Would a warlock 12, valor bard 6 have an attack action of three pact weapon attacks (with thirsting blade) with an ability to exchange one of these for a cantrip from Valor bard's extra attack feature?

So 3 attacks or 2 attacks + a cantrip?

This is one I have been pondering, I'd say by RAW, this doesn't work and it's due to the wording of Devouring Blade, Devouring Blade which grants the 3rd attack specifies that it only works with the extra attack of Thristing Blade, it's quiet specific in this wording and so I do not believe you can use Devouring Blade on the bard's version of Extra Attack as it's notably different to Warlock's Thristing Blade. Bard's ability to replace an attack with a cantrip is limited to their own version of extra attack, I would say.

Now Eldritch Knight, can replace an attack with a wizard cantrip, it's not linked to fighter's extra attack and so should work.... but at that point... why not just continue on with fighter levels and additionally it's specifically wizard cantrip.