Looking at that Solved Plugin, from what I can tell it only allows selecting one answer as the solution, right? IMHO that is reducing what makes StackOverflow and the whole StackExchange experience great to only a very slim part of it.
As I see it, the true advantage of StackExchange is that it involves the community in a really great way, recruiting active community members to be kind of curators and moderators. It is preventing duplicates and unanswered questions by putting the spotlight on similar questions and the unanswered questions, allowing to give bounties for answering them. And many times I did not find the best answer by looking at the one selected as the solution, but in the 2nd answer which sometimes has more votes than the selected answer (or even the 3rd). So my fear here is, that this plugin is too simple for the very complex beast that is Q&A.
I also second the concern that you mentioned about overwhelming the regular forums. I think if the merge is the choice here, there needs to be an effective and simple solution for people to opt-out of the Q&A, like a checkbox “show/hide Q&A” or a separate view.
I voted “Something else” and here is my idea: Why not create an StackExchange site for unreal engine that is part of the official StackExchange network? Yes, this would give up control but this is THE industry standard for anything Q&A.
Integration-wise that would be difficult I guess, but there’s also StackExchange for Teams. Since Epic is pretty big player in the industry, maybe something can be negotiated with the people behind StackExchange here? No need to reinvent the wheel.
Same with the Wiki. Instead of Epic going their own way, there already is a great community wiki. Why not approach these people and join forces to the benefit of all? IMHO the best things happen when companies openly approach their fans and invite them to collaborate, like the engine contributors on GitHub 
“If you love something, set it free.” 