I may have done something a bit wrong, I opened the door for Lorenzo and killed him before I realized you could kill him w/o entering his room. Anyway, a week later, Jack Cabot wants me to return, presumably to get Lorenzo's Artifact gun, and when I get to Cabot house, there is a quest marker hanging in the middle of their living room, but Jack won't engage in any relevant dialog with me.

The console command 'GetStage' showed me at 2000, which is the end of the quest.

I tried a 'CompleteQuest' which did nothing, and a 'completeallobjectives' which didn't help either.

If I do a ResetQuest, will I be able to start the quest over from the beginning?

well yes, if you do reset quest, it will resart it. try respawning cabot. in any case, you could spawn lorenzo's gun. =P