In my current run (patch7 with mods) I noticed that Marcus is there as soon as we entered the last light inn and triggered the dialogue with Jaheera but couldn't approach Marcus after that. I wanted to fight him before his raid to kidnap Isobel.
My only successful option at this was to: trigger the dialogue with Jaheera, switch to another character, let him go invisible (anyone gets bounced out of the area as soon as they get spotted inside the area at this point), go near Marcus, use the improvised melee weapon to move him out of the LLI and eventually (for extra safety) teleporting anywhere else while carrying him, he teleports with you. Nobody cares if you do this as long as you interrupt the action while you are still walking and carrying someone that is not hostile.
At that point we could attack him while nobody saw us and kill him, I'm not sure but he seemed to not get hostile while everyone of my pary took part in the execution: we silenced him and blocked him with block person spell (I was afraid of his fight so I went hard). But I may be wrong and he gets hostile as usual npcs.
Switching back to the dialogue with Jaheera everything went fine, I think you can involve Marcus depending on choices at this point and even expose and kill him but I can't remember if I could choose doing this..
Marcus disappeared, got killed, nobody knew or asked, like he had never been there.
After this everything in LLI goes as normal but Isobel is inside from the beginning and there is not her cutscene on the balcony, there is no assault by Marcus and his minions or someone else in his place, she has her normal dialogue and gives her blessing as normal (fun fact: she referred to Jaheera sending us to her but this never happened because I ran upstair to meet Isobel as soon as I could ignoring Jaheera downstair).
Because there is no assault nobody dies in that cutscene, I don't really know who but I remember someone gets killed and/or kidnapped by demons before the fight begins.
I don't know if something brakes because of this in the future because I don't know if the kidnap attempt takes place if you expose and kill Marcus in the dialogue ouside.
