Yes, I would like to know to why 2.3 worked and 2.4 does not.
Since I don't have a 2.3 installer anymore, I can't compare things. But even if I could, I'm no gauge mechanics, so my analysis would be not more than skin-deep.

Glad to hear that the 707 on 727 thing works. I had around four flights now and apart from the mentioned "ID already exists" messages for some seconds after loading, everything is there and working, even with the clearer 707 click sounds, so the fidelity even entered a new level.
Surely not to be called solution this whole thing, but I'm ok so far since I get a sounding 727 on the first load which doesn't mix up my offline ATC solution.