The game has been updated over time and certain area's have been revisited for "touch ups" so its almost impossible to tell if anything is bugged or not without actually reading the save data (im not willing to do that as rpg maker saves are too complex for my brain on most of the days) the best case scenario would be somehow fucking with all the flags and switches and making it work (which is something i would advise against as the game does seem to fuck up when you toggle something that should not be toggled at the time)
I skipped over the conversation that you had with the other user (mainly bcs i don't read anything that is not remotely "i need help") so i may have not noticed that you had certain flags on (my mistake like always) so now that i actually got some degree of my brain back (due to sleeping for like 8h) i decided to check my own saves and see if i could not recreate the same issue and came to the conclusion that i do not know what the fuck is wrong with your issue, i have tried different saves in different time frames (some of them being from 0.6 version and some from 0.5 and the most recent one, none of which caused any issue)
With that tested i decided to see if the issue was just poor timing (as certain events become obsolete when bigger events trigger) as that could be the case with this one, so the next move was to check if the 2nd gathering, incubus war or anything else would "override" the event, and they do not (as they lock you into a specific part of the game they don't "collide" with the event itself)
Next on the agenda was testing if you can skip over talking to kara and having the event bug out, (which somehow bugged my screen and forced a black screen due to missing flags ._.) and aside from that one little fuck up, everything seems fine and nothing should actually block you from getting kara + orcent (something i noticed is that the event does bug out only if you are missing every single flag, what also happened was that i somehow got kara to talk with orcent and riala even though she was considered dead in the game, so i got no fucking clue how that works now)
Next was checking a fresh new save and forcing the player character to spawn inside the HQ (instead of the beginning of the game) which proved to be very annoying as i had to flag everything myself BUT it allowed me to have a controlled environment in which i could test just the kara path.
So i got lucy (because she is a key component for this) and orcent being back to HQ (so i can actually interact with him) and having kara recruited (having the dead/alive flag changed nothing as it only serves as a "key" for her recruitment that happens during the war) so that lead me to believe that the main thing missing would be the only thing that i have not touched yet... orcent conversations (which i set to 12) and that put me in a state of "kara being next to orcent, them having talked to lucy and no sex scene happening"
so the last thing on the check list was forcing the riala/orcent convo to happen which is seems to be the last thing missing (and since i do not know what triggers that specific event, i just brute forced it) and with all that...
I got kara lucy and orcent to talk, i got 12 conversations and had kara + orcent on, then i just walked to the main floor and the whole thing triggered (so you may be missing riala/orcent convo somehow) as its the only trigger for the sex scene
Starting a new save should eliminate this problem (because it just seems like the save ya got is just borked) but you can always upload the save and i can simply brute force shit till it works (that method is like last resort for things as it may break future updates)
The timing for the scene can happen at any time that you got kara riala and orcent (+ lucy) in the HQ so you most likely should be able to trigger it before/after the gawnfall synod as it unlocks some areas and such.
To explain things so you dont get confused, common event is as the name suggest, a event (just like any sex scene) it allows sierra to write a huge wall of smut in a tidy window in the editor and have it be called when she wants (you can look into them when you decrypt the game and run it in the rpg maker vx ace) (attached the picture of the menu with the event selected)
Another thing i should explain is that you only need 3 requirements for the scene to trigger (which i attached as a picture too) and the scene should play on its own when you get to main floor, so you are 100000% either missing one of them Or the game took a shit and somehow the data is plain fucked (can happen on very very very rare occasions when a game is updated but the old "code" in the save decides to not give way and it just eats shit and corrupts itself)
And now that I think about it, this is literally my first playthrough. I've been essentially updating/overwriting the same save file that was first made when the game didn't even have some twists like
I'd suggest not overwriting any saves in any rpg game (it mainly just makes you waste time when something fucks up or a boss decides to sit on you and insta kill you ._.)
I will definitely play the game again for sure (and min-max the shit out of it too), but only once it's completed. It was...IS enough to pick it up every some years and binge the living fuck out of the available content ONCE, I ain't doin' that again lol
booooo min maxing booo anyway, the game should be done this year (aside from art which can take tad bit longer than anyone would like because drawing smut takes time) hf with 150h+ of content (if you count the different routes you can take on certain events)
