Their engine should definitely be able to handle a reaction system as it'd mostly just be adding a pause and then allowing player to to choose something. I think it'd be less having to remake things and more just adding to what is already there. But this might be too generalizing or completely and utterly wrong.
I hope so but I suspect the reason we aren't seeing any reactions (attack of opportunity) is because Larian doesn't really have a good solution. It's probably why we haven't seen paladins yet as a major part of their core powers is reactionary smites.
But I lost hope with Larian after their patch 4 reveal. What we are seeing is what we are getting, with only minor changes.