I'm not sure i agree. You don't put retained salary on LTIR, you put a player on LTIR, you get the ability to call up replacement player or players who's salary may not exceed the amount of the Player Salary and Bonuses of the unfit-to-play Player who the Club is replacing.
Retained salary is still the players salary, it's just permanently paid by the retaining team for the life of the retained salary spc,
I suppose it's all hypothetical until the NHL comes into the situation and has to interpret their own CBA, in the end, they may come to a conclusion different than you or I, not sure how often, if ever, teams reaquire players they've previously retained on, so it's unlikely to come up.