As a subclass feature? Sure.
But not buried in some arcane corner of an ambiguous multiclassing interaction.
I am still thinking of Fast Hands as a subclass feature as well. Until there is a clear consensus on how that works, talking about multiclassing is (to me) beside the point. (And, frankly, I think you are making the best case possible for the view I am now disagreeing with.)
Possibly, the whole matter will be solved, clearly, by any discussion of scrolls in the DMG. For now, we're doing our best with what we have. You may be right, and if so, I'll be happy. But given what we see in the PHB, I have to believe that reading a scroll with a spell on it, which casts that spell under special circumstances (the material components are paid when the scroll is scribed, for instance) is using the Magic Action: it's not any of the other actions listed, and i don't believe it's some separate thing that is not an action.
And the Fast Hands ability lets a Thied do what is normally an action (a Magic Action, in my view) as a Bonus Action.
Exceedingly poor design if you hide your best options under a few layers of optional rules that aren't clearly explained or signposted. WotC is capable of mistakes, but this would be something truly disappointing in its presentation.
Arguably not as egregious as failing to spell out how nick works with light weapons, and leaving it to be inferred from the interaction of three or four relevant class features. Yes, there are failures or presentation I think they should have caught. That doesn't prevent us coming to a consensus view of what the rules say (even if it's not what they should say).
When this thread started, I think I agreed with your position (i.e. I agreed with what I see your position to be). But I've been persuaded that's not what the book says.