A U-turn solution would rank at the bottom for me, there has to be a better solution.
The OP say you have 16" I-joists, are the flanges 2x or 3x? Even if 3x, that would leave 11" of web between them. And a 2" pipe is 2-3/8" tall, so the difference is 8-5/8", which is the most a 2" pipe could fall while staying within the web. That would be enough fall to travel 34' 6" at 1/4"/foot. Is the tub drain really approaching that length?
I guess it's connecting to a horizontal 3" drain, so I should subtract 2' 3" from that travel distance to get 32" 3" maximum theoretical horizontal. And that assumes the I-joists are in a level plane; if they are out of level going against you it could be less.
Also, is that a freestanding tub? Model number? Does it have an overflow that gets connected below the floor, or is all that above the floor? Because with a underfloor waste and overflow connection, the trap couldn't be so high that the outlet could hit the top flange of a joist, I would think. But if you have only a combined waste and overflow coming through the subfloor, then I see that you could have the trap higher.
Can you pull a vent off the tub drain in the right-hand wall? I see that wall is over an I-joist, but the vent could rise at a 45 degree angle, just kissing the corner of the I-joist flange. Depending on the wall finish and possible baseboard, a 1-1/2" vent might be hidden. Or you could slightly move or fur the wall to hide it.
You also have the option to add a small wing wall on the shower, so the top side (on the page) isn't all glass, and run your vent pipe up that wall.
Wayne ... first thanks for taking the time for such a detailed reply.
Yes, u-turn is not ideal, but I was hoping to stay in the one joist bay and save boring 2 holes in the joist to the right of the tub drain just to pick up the vertical vent in the stub wall, and losing the 1 1/2 inches in height for the joist flange. I attached a drawing scenario 6 to show what I was thinking.
I can still go with drawing 5 and go over one bay to avoid the 180 degree u-turn, I think I have just enough height, but because I will lose about 2 inches to avoid cutting the joist flange, it will be close.
I do have a 16 inch joist bay, however, where I would like to enter the main house is through the rim board just above the top plate on the house/garage wall (lower left corner of drawings), which is about 4 1/2 inches from the bottom of the flange (or 6 inches down from subfloor). See attached picture
. This is in a closet back wall in the main house, so I do have the option to drop it down lower into the garage space and go in through a stud, lower, however, this is just to the right of the main house/garage entry door, so it would look a little ugly, but I guess I could frame in a chase around it for aesthetics. I would prefer to stay in the ceiling/floor cavity of the garage if possible.
The tub is freestanding/claw feet, Bain Ultra 6636, and has the overflow and drain above the subfloor, so only the drain pipe coming through the bottom of the subfloor. Shower will be a 60 x 36 pan with a corner glass walls, so no alcove wall to work with unfortunately. The room
right wall is directly over a joist, but if I have to jump a joist bay, I have the 45 degree stub wall I can get a vertical vent in and eventually up the right wall above the bottom plate.
I think I answered your feedback questions, but if not let me know. I may do some more calculations, but if I stay close to the flange on the right joist (which Boise says you can actually cut the i-beam web right to the flange, as long as the flange is intact), I should be able to make the slope work with drawing 5.