Well the FO has said they wanna build around Lauri. If that is true then most of your logic goes down the toilet and they only really have 2 options:
Either they draft with the fit as a high priority or they make some trade(s) to either move up/down or to acquire a guy.
But I guess your argument is that their intentions are fake and it may be so.
It's not that their intentions are fake, although I do think it becomes more likely as time goes on that building around Lauri is less likely. But that's not really the point.
My logic is this:
1) Having two players that can play together, but still leave holes on the roster is not the same thing as having two players that cannot play together. The former matters, just not nearly as much as the latter. Lauri is one of the easiest fitting players to begin with, so no matter how much you consider him it's not really a huge deal.
2) You do not have to put all the puzzle pieces together immediately, the Jazz roster isn't going to be set after this one draft. For example, the Jazz need a ball handler....but that doesn't need we need to draft the best lead ball handler. If you think Flagg and Harper are on the same tier without much separation, that is when I think you can start to think about fit. But if you think Flagg is better than Harper, I wouldn't draft Harper (or anyone else who you don't believe is as good as Flagg) because that player is a better fit. Flagg would still leave you with a need for on ball creation, but you don't have to try to resolve that through the draft.
I don't think they're in this dilemma that you presented where they must draft for fit or trade. I think they can simply take the guy they believe is the best player available wherever they pick.