The way I did this quest and had no bugs: got invitation from drunk dwarf woman, was allowed in through the shop entrance; spoke to the groom first: I had Ifan in my party and the groom asked him to help him with the assassination, Ifan refused. Then I spoke to the bride Isla and then with her dad upstairs. He gave me info about queen Justinia (but I did not insist a lot at this point. Had success with persuasion tests using 6 Persuasion + higher stat. Then I went to the garden and exploded the cake (dad mentioned his daughter did not even got a chance to taste it). Reported about the cake and learned that it was a gift from a doctor. Took the note left next to the cake (mentions Isbeil and updated log). Then went to the grooms house which is next to the the old moneylender lady's. Found his diary in the bedroom, opened hatch, visited the lab in the basement, escaped by activating the explosions. Back to the merchant's place, the groom was still on the balcony, I left Ifan in the other room, just in case I lose pts with him when I accepted the assassination contract, got xp at that point. Then went inside and told everything to the daughter who believed me and run upstairs to tell her dad. When I talked to him he gave me only 2 options 'oh, you came to do the job' and 'take your leave'. I chose the second and as I was leaving he stopped me and asked me to kill his son in law instead, which I accepted. I got to the groom's house (he had left the balcony and was in this time, in the basement lab. My main is an undead and heals from poison so I left my party in the first chamber where there's no traps and fought the groom 1 to 1 (it was a bit of an annoying fight because he teleports and drinks potions all the time and regains health & armor but I eventually got him. Back to the merchant's house I got xp and a choice of gear for reward. The wand that the groom drops had only 2/5 charges of ooze blobs when I looted it. There's also some good stuff in a chest at the end chamber of his basement lab.