we're doing exactly what you say (by coincidence). He's quarantined every night all night and was quarantined for 3 weeks (2 weeks initially and at least a week when we first noticed the peeing issue). Other cat doesn't lie on my daughters bed or desk that we've seen (had him for 12 years)... and daughters room door is closed at all times (closed it after first couple of pees on her bed... mattress also soaked in anti icky poo honestly). He snuck in while the door opened... looked like crept open on its own (also have a baby gate on her room) and hopped the baby gate..
You mention the down comforter, can you try a down alternative for a month or two? And some cats like soft cushy places like comforters to pee...I've heard of using a larger flat sheet over the TOP of the comforter and tucked in smoothed so that it is not a rumpled comfy surface. If he's not peeing while your daughter is IN the bed, this could well work. I'm guessing you've put waterproof mattress protectors on the mattresses now?