In case anyone's interested, I believe I figured out why the bug with the Sith party and Sarna is present - seems it's not the author's fault.
I spent ages comparing the the author's modified tar02_sarna021.dlg with the game's, and trying to figure out why it was so different; most notably, why the party tree had been broken.
As it turns out, the game has two copies of tar02_sarna021.dlg.
The first is in the area tar_m02ad (the North Apartments) and includes the correct party dialog that causes the partygoers to collapse, the quest to finish, etc. It also includes a bunch of dialog with Sarna that centers around buying a Sith base passcard off her, which, afaik, wasn't part of the final game.
The second is, iirc, in area tar_m02ae (I assume the Upper Cantina). It includes all the dialog you have with Sarna when you meet her in the cantina, and for some reason also includes a broken party dialog that doesn't fire the right scripts and gives you a Sith base passcard.
Now, if my guess is correct, there must have been some original scenario where you could pay her 2000 credits (according to the first dialog instance) and go the party to get a passcard off her. I guess that must have been ruled out in the end (afaik, buying the droid to break into the base is the only way?). Thus, all the other dialog was just left in, and isn't a problem, since the game uses the file with the correct dialog when in the cantina area, and the dialog with the correct party scripts when in the apartments, and ignores the rest.
Now, the point of all this. Since the author modified tar02_sarna021.dlg from tar_m02ae, and correctly so, since that's the dialog that's used, the broken party was included from the original. Thus when the game tries to load either of the originals (including the apartments one), it finds a file with the same name in Override, and uses that. In other words, the file is overriding both the originals, when it was only intended to override the one for the Cantina. I guess this is a problem with the behaviour of the Override directory system.
So, if you were to replace the correct copy inside the RIM file, the bug would disappear without any changes to the author's script, at least in theory.
And if that was entirely incorrect, have mercy. I only started looking at modding for this game yesterday