Things didn't quite go according to plan. The loopback I'd set up in Plan B merely meant that the "Spanish Admiral's Son" quest immediately detected that you're in St. Pierre tavern and started the self-dialog again, repeatedly.
So here's a version that does work. It turns out that Billy Brock is already in some sort of "guardian" mode, in that he stays there and you can talk to him, but by default he just tells you to get lost. Setting quest attribute "Lucas" to "sign" tells Billy's dialog to go to the bit which would normally happen after you've done all the self-dialogs, then he talks to you, leaves the tavern and leads you to the kidnap house. So all I've done is add a check to look for quest attribute "gambling_with_girl" having been set to "to_FDF", in which case it simply sets "Lucas" to "sign" and then exits, leaving the scene clear for Virginie to do her stuff. When you've finished with her, go back into the tavern and talk to Billy Brock. It's not perfect, it would be nicer if you could leave with Virginie and then return and do all the self-dialogs about the secret sign, so if anyone wants to do a better job, feel free. Meanwhile, at least this breaks the deadlock caused by both quests wanting to do things in St. Pierre tavern, and if "Spanish Admiral's Son" is the only one active then it does work as normal.
@morgan terror: if you copy this into place, and provided nothing else has been done to "quests_side.c" which depends on later updates, you should be able to play from your savegame outside the tavern. I saved game myself as a prelude to experiments on "quests_side.c" (can't use yours as I'm playing Beta 4) and was able to try out various things before settling on this.
Other notes:
The other kidnapper, Snorri Baldursson, has a dialog file which wants to exit to "Lucas_sign_fight2_1", which triggers an error message because that doesn't exist. Normally you don't get to talk to him because Billy Brock does all the talking. Perhaps give him some new dialog which tells you to talk to Billy Brock - the only way you're going to see this is if "Girl Won in a Card Game" is active as well so the normal ransom scene hasn't happened and both the kidnappers are still standing at the bar waiting for you to talk to them, so he may as well tell you to talk to his boss.
Virginie d'Espivant is most ungrateful. After you've suggested she returns home, she stays in St. Pierre and if you talk to her, all she says is "Leave me alone!"
Perhaps set her location to "None" so she disappears entirely, which is probably what would have happened if the original author of this quest had remembered to do it rather than leave her stuck outside the tavern. Alternatively, I seem to recall seeing some talk on the forum about using her as an officer, which isn't going to happen normally because she's protected by 'SetCharacterRemovable(characterFromID("Virginie d'Espivant"), false)'. If she's not going home as originally suggested, perhaps now she's available for hire as an officer.
"Lucas Da Saldanha_dialog.h" could probably do with a slight alteration. "Lucas Da Saldanha_dialog.c" currently does this when you've paid the ransom and are talking to Lucas outside the kidnappers' house:
Code:
link.l1 = DLG_TEXT[7] + GetMyFullName(PChar) + DLG_TEXT[8];
DLG_TEXT[7] is "I'm captain ", which means if you already have a rank then this is going to look silly. In my case, "I'm captain Lieutenant Horatio Hornblower". Perhaps remove "captain" and let "GetMyFullName" add the relevant rank, if any.