Where does the P-trap go?

I almost forgot, I have visually verified that there is indeed a trap at the bottom of the riser for the bathtub. So I will be connecting the tub to the riser directly, without a vent or trap.

Thank you to those that provided helpful comments and insights. You got me pointed in the right direction. It's very much appreciated.

The Bornoulli Principle uses "velocity" NOT volume to create the "negative pressure". I have a customer who had a pump installed for a room addition. They connected it to the 2" vent for the toilet. EVERY TIME the pump operates the water flowing past the toilet's 3" connection "SUCKS" the water out of the toilet, EVEN though the 3" line is NOT full of water. I love it when people tell me I do NOT know what I am talking about, because that just affirms that I do. "Wet venting" has NOTHING to do with whether the toilet is the first fixture or the last one. IT depends on HOW ALL the piping is arranged, and it is on a "fixture by fixture" basis. In fact, you could have a "lot" of wet venting, and NOT involve the toilet at all.

Wow, thats some impressive suction... hj, would you mind posting a simple sketch of this setup? I'm just curious, and not sure I fully understand the layout you described. Thanks.

I think there's no more point in trying to convince this guy, he's obviously made up his mind. It'll be his problem if/when his bathroom is full of sewer gases.

The NASA guy thinks he understands it better than the life-long pros. I'd still like to hear that engineer-speak description of the overflow functioning as a vent though... that cracked me up

I would fix it for reasons other than sewer gas, like for example, peace of mind, pride of work.

I couldn't cover up a hack job with a straight face and call it good, call me crazy.