I disagree with this line of thinking. I think they had him in a very important role on the team. They counted on him to take on some of the toughest minutes on the team in terms of zone starts and quality of competition. They also gave him plenty of PK time. The only thing is they gave him a terrible partner to help him full fill that role.
Ask yourself this, who else on RD could be counted on to go up against the other teams best and push play from the dzone into the ozone consistently. I would argue only Buff but his elite offensive ability would be neutralized a bit in such a role. They only trusted Myers to play a middling role with a good partner. He
If I was Trouba I would have an issue with being tasked with performing the role they asked him with an inadequate partner and perhaps not getting more pp time. I don't think the Jets have been trying to marginalize him at all and I find the talk of sandbagging him silly. I think they valued him to play one of the more difficult roles on the team with less help because they knew he was capable of it.
I think you're giving the Jets staff slightly more credit than they deserve. Your scenario of giving him an important role presumes that they were actively monitoring all stats throughout the season. Tbh I think they overvalued Stuart as a "shut down defenceman" and mentor and put Trouba with him purposely because they initially thought Stuart would be beneficial for his development. Then, they realized too late that the ship was sinking and decided to rearrange the chairs by giving him some offhand time with Buff while keeping him anchored to Stu.
I also don't buy the whole QoC argument. Sure he faced some tough competition, but I don't think it was entirely the Jets choice. Think about away games where Jets have first change. If I'm the opposition and I see Stuart on the ice I'm going to likely put out my best scorers if I can. I'm thinking that the Jets cared more about who is on C rather than D in the dzone in these situations. I don't have the numbers in front of me, but my hunch would only be plausible if something like the home Rel Corsi % was better than away.
I also wouldn't ask "which RHD do I want out there". I would look at pairs, and it is clear to prefer both Buff/X and Myers/Enstrom over Trouba/Stu. That third pair needs to be played, however, so where might they best succeed? If I'm the Jets I'm thinking that if Trouba/Stu are in more defensive situations, I have the insurance of my better centres as well as less demand for back-checking.
We can both agree that the hardest part of Trouba's role was having Stuart as his frequent partner. You also make some good points about special teams and QoC that I'd want to think on a bit more. I don't think they were sandbagging Trouba per se, but I also don't think that the Jets believed they were giving him an important role (they might have been selling it that way though). My read on the situation over the last few years is as follows:
Jets: Stuart! So leader, much shot blocking! He'll help this young D become a pro!
Stuart: hurr durr
Trouba: I think I can, I think I can...
Jets: Our plan is working!
Stats guys: rabble rabble
Jets: Oh noes! But wait.. Trouba is keeping this thing afloat. Maybe he won't notice. *throws offhand bone*
IMO, by the end of it Trouba's role was to mitigate damage from his partner more than it was to shut down top lines because, as you say, he was capable of it.