The way I see it, its about how much risk you are comfortable with vs. how much capital you are willing to pay. Its a balancing act really. If Tua's future is somewhat nebulous, you might absolutely get him at 5.
The real question is, would you rather Tua's future being more positive and pay 5 + 2nd round pick for him. Or would you rather have Tua moving around throwing darts before the draft and have to pay #5 + #26 for him.
These are obviously just fictionnal examples, I think its pretty fair to assume that the price the Fins will have to pay is directly associated with Tua's health concerns. The good news here is that there will be less risk involved with getting Tua on your team. If instead the news was that he had a setback, then the good news would be that he might cost less...
I know you mentioned your examples being fictional but putting more reality around them is important when trying to compare which is the most ideal.
Tua has a setback - Stay at #5 and we can likely get him. Moving back in the draft would be dangerous given the teams right behind us that would be interested in spending their 1st pick on him and having him sit for a year.
Tua does not have a setback but also not fully functional by draft time - Likely need to spend #5 and #26 to move up to #3.
Tua throwing dimes and moving around freely by draft time - Likely need to spend #5, #18, and a 1st rounder next year.
The reality is that teams are able to secure a ton of current and future capital when willing to move down from a spot where a potential generational QB is available. See below:
Eagles move to #2 and give up
#8, 3rd rounder, 1st round following year, 2nd round the year after (and I would argue Wentz had more question marks than Tua)
Redskins move to #2 and give up
#6, 1st rounder following year, 1st rounder the year after
Jets move to #3 and give up
#6, 2 2nd rounders, and a 2nd rounder the following year (same with Wentz, Tua more generational than Darnold by a wide margin)