Question about plumbing vents

Hi everyone,

I've been having a bad odor in my bathroom since a recent renovation and it seems to be coming from the tub. Through testing of p-traps, and looking at old photos taken during the bathroom renovation, I have determined that the plumber, who did the renovation, connected the drain line and vent pipe of the sink to the drain line of the tub at a point before the tub p-trap (shown in photo 1). I don't think this is to code, but by itself it doesn't seem like it would cause any major issues. (The drains all drain appropriately and without any gurgling).

What seems to be causing my issue is that a separate vent pipe in my attic connects to the sink's vent pipe, and so sewer gases can flow directly out of the tub's drains into my bathroom. (also shown in photo 1).

I know the appropriate solution is shown in photo 2, connecting the sink drain / vent pipe after the p-trap of the tub. However, that would involve opening up the wall and potentially my bathroom floor. So I'm wondering if it would be completely unreasonable to disconnect the second vent pipe my attic and just attach an Air Admittance Valve? (shown in photo 3).

Photo 4 shows the actual plumbing (it's the only picture I took post plumbing work. I have circled the connection of the sink drain to the tub drain.)

Photo 5 shows the vent pipes connecting in the attic.

Context: There are two bathroom separated by a wall, and the drain from the sink in bathroom 1 is connected to the drain of the tub in bathroom 2. The sink and toilet in bathroom 2 seem to be operating fine and using a different vent.

Thank you for your help.