Another question that'd be interesting for me: are you able to leave the painter's room or is it invisibly blocked?
A script command via console has a good chance of working, but you'd need to be aware that any saves you make afterwards won't work, so it should be the last thing you do in the mission. This is no big matter, since it's only needed to tick off an objective and it unlocks a 2nd flavour conversation if you return to him later.
These are the 2 console commands, in order:
script painter2_finished_conversation_end1();
script painter2_choose_conversation_end1x();
To play the 2nd ending conversation, leave the building, then return to stand in his field of view and use the below command. Since you had already dealt with Marlow for the 1st conversation, the 2nd one will be quite short:
script painter2_choose_conversation_end();
In case this stuff doesn't work, you can find transcripts of the conversations at the bottom of the mission's collab document. The conversations go differently depending on whether/how you've dealt with Marlow.