I can't quite keep track of your entire system across multiple threads and with only limited photos.
But for the downstairs half bath, where the lav and WC will ultimately join a stack carrying drainage from the story above, for wet venting you need:
- A dry vent for the lav. Under the IPC, this can be a 1.5" vent through the roof, or an AAV.
- The lav dry vent has to come off the lav trap arm within the appropriate limits--i.e. if the lav trap is 1.25", before the trap arm falls more than 1.25".
- Then the lav fixture drain (fixture drain = only one fixture on the drain) joins the WC fixture drain almost any way that drains are allowed to join (*). The vent for a WC isn't subject to elevation restrictions.
- Then the branch drain joins the stack.
(*) Because PA has adopted the IRC's plumbing section, there is a limit on horizontal wet venting--you can't join the WC and lav drains with a fitting (combo) where the lav comes in horizontally, the WC comes in vertically, and the outlet is horizontal.