Have a slow leak on my backflow preventer. I replaced the bonnet and added a new o-ring, but I still have some slight seepage coming through. Any idea what could be causing this? Would plumber's grease or a silicone sealant on the o-ring and threads of the bonnet help?

We are remodeling the bathroom, moving the sink and bathtub (originally a shower). The shower and toilet were along the same pipe line and vented in a privacy wall next to the toilet. The sink used and AAV. I can continue to use AAV in the new sink location or tie all three into the same vent (if possible). All three walls that could be used for venting are structural walls and cannot be used for venting (as far as I know). The one internal wall has a triple joist beneath it so cannot be accessed directly. Based on my image showing the placement of the sink, bathtub and toilet, along with the existing plumbing locations, what are my options for venting? Am I able to vent with just one vent? Will I need two separate vents? What locations are available to me that don't require the privacy wall? I figured the shower would have to be vented in the adjacent wall by the drain, but I'm unsure about the sink and toilet. If I doubled the thickness of the wall behind the toilet (added another 2x4 wall) I could vent in that "false wall", but it would visually look odd having just that section twice as thick as the section on the other side of the door. Any options and advice are welcome! Thanks! [url=https://drive.google.com/uc?export=view&id=1-s6G9eTu-rNU6UrgXmT0ttg9pOssgqGf][/url] [b]CLICK THE PICTURE TO SEE MORE DETAIL[/b]