With the current writing it's absolutely impossible in my opinion, I agree. If the writers would have had this idea from the start and would have written this development from act 1 on, it would have been feasible to have several of the current companions feeling this way in act 3. Because they only added Halsin's romance late in development and added the poly romance option specifically for Halsin so late in development, there's no way to organically write Shadowheart's change in views on relationships between act 2 and act 3 and make it make sense.
A popular opinion is that, with different writing in act 1 and 2, both Karlach and Shadowheart could have been written to be okay with such a relationship for example. Karlach's current dialogue in act 1 even contains "I know you and hairdo girl have something happening, but keep a piece left over for me, hmm? I won't always be too hot to touch.". That's still by far not enough within the current writing to make a poly romance between them and your Tav/Durge believable, especially considering the ultimatum in act 2 that pushes for exclusivity, but they had a little bit of groundwork there if they really wanted to rewrite companions to be accepting of poly romances. But the fact is that they didn't rewrite anything concerning that in act 1 and act 2, they only added things in act 3 and only specifically for other companions together with Halsin. And as there is no groundwork for any companion in act 1 and 2 for this to become a thing in act 3, to me it completely misses the mark and makes every companion involved in this seem out of character.
