As grim as the situation might have looked as recently as today morning, the major scene issue is now solved, thanks to a strike of pure luck, though credit is due to jc321, who helped me confirm the issue wasn't with just my machine, and fishburger67, who has unknowingly pointed me in the right direction back from the year 2022. While scouring the Internet for the 100th time with the same question, with a slightly different wording, I happened upon his post concerning a fairly similar issue, another fellow modder was having, and he prompted this guy to verify the quest's aliases. Though this turned out to be for a different reason then what, in the end, turned out to be the issue in mine, it put my eyes where they needed to be, and I noticed that there are two unfamiliar aliases, not involved in my scene, which I completely forgot about. Pointers to ZAZ scripts I was trying to resolve the issues I'm having with disabling the player's movement after SL animations, which I completely forgot about. The pointer aliases only need to be in any quest that is start-enabled, so there was no particular reason for them to be in this quest. So I made a new quest, moved them there, and voilà, the scene suddenly starts.
I'm still getting the occasional validation error in CK, but other than that, things are looking up again.

Now, why the Patience potion, you ask?
While all this was unfolding, I got a call, and I got another big job, from a new customer, so all work is halted. I will still try to at least upload the unfinished version for those who can't wait, as I have promised, but since I have to make a living, little further work will be done until the middle of January. Sorry guys. But also, yay!