Nothing is more entertaining than fans overthinking a structure that doesn't actually exist. #justdontcare
Nothing canonically indicates that Jack is second officer. Canonically, he's an ensign without portfolio.
Well you're more educated than I am, because I didn't know "ensign without portfolio" referred to anything other than the lowest-ranking bridge officer who hasn't been given a particular departmental or bridge console assignment. (I just took the phrase "without portfolio" from the phrase "minister without portfolio," which is a term for U.K. Cabinet members that haven't been put in charge of particular departments). Maybe Jack could have the position Worf held during S1 of TNG -- he was basically a bridge relief officer who stayed on the bridge all throughout his shift and took over at major console stations if the regular officer was called away. He would also sometimes stand in as officer of the watch if the regular members of the chain of command were unavailable.
Oh, I don’t think there’s any danger of that. srsly, I had heard the phrase “without portfolio” before, but I had no idea what it meant.
Worf's S1 job was supposed to be Officer of the Deck, a very real and legitimate job in navies of today. Basically, they're the officer directly in charge of the bridge, and if the Captain or XO have a problem with how the bridge is run, the OOD (as it's abbreviated to) is the one who answers for it. The job typically gets ignored in Star Trek or anything set on a ship in general due to a desire to be narratively streamlined. TNG only used it in S1 because Worf was a late addition to the cast and all the other positions were already occupied, which is why they seem to treat the job as an afterthought. For TNG's S1 writers, it literally was an afterthought.
Still sounds too senior for Jack then. (Memory Alpha only lists Worf's job as relief bridge officer though -- no mention of being OOD.) Anyway, if I were Terry Matalas and Star Trek: Legacy got greenlit, I would probably have Jack as bridge relief officer.
IDK that it's too senior a role for Jack necessarily, depending on the ship OODs can be anything from Lieutenant Commanders down to Ensigns quite easily (though Ensigns and JGs will sometimes stand as the Junior OOD/Conning Officer under a senior OOD on major warships), for that matter I believe it's possible for qualified enlisted Coxswain to stand as OOD on minor vessels (though probably more common in the Coast Guard than the Navy for US service). I agree that Worf wasn't the OOD, given that he appeared to the most junior member of the bridge crew, whereas the OOD is typically the most senior (command rated) officer on the bridge with the exception of the CO or XO (who IRL would basically never be on the bridge at the same time).