Thanks for your comments Wayne.
Attached are some pics/diagrams that hopefully provides more information and gives a better idea of the whole thing. The double joist at top of the view from downstairs (a bathroom, whose ceiling I opened up) is where the back wall of the upper bathroom is connected.
Addressing your questions/comment:
With respect to moving the 3x2x2 wye farther downstream, I can't as I don't have enough space (see upper right on photo).
The 3" horizontal line is carrying on the lav drain connecting on upper right. The toilet is upstream with a 9' run, hence the need for additional venting. See view from below and options I am considering? And yes, that I-joist hacking is really bad, see more general info below.
This is an upstairs bathroom that was rough plumbed (by certified plumber, also inspected) several years ago, to be finished at a later date (ran out of money at the time). The rough plumbing was for a toilet (where the i-joist is badly cut), a vanity, and a shower at the lower left corner of the picture. I removed the toilet flange and ran the long elbow instead)
I am now finally working on it and changing the location of the shower and toilet, shown in diagram, at the insistence of wife ... Instead of shower at lower left corner it will be a toilet, and shower will be close to the original toilet location was.
As far as venting the toilet, my plan since it is 9' (too far, > 6') from existing vent was to add a 2" vent off the bottom right corner (about 3' from toilet, showed in orange) run vertical leg in the wall, then connect to the existing vent above the ceiling. This would also vent the shower since it would be just 2' from the trap.
But a friend of mine is saying I don't need to bother with that, I can just use the (old) shower drain (dotted blue line) to vent the toilet (it would be 5' from the vertical drain) and I don't need to be worrying about venting the shower since it connects to the 3" drain that is already vented at that location from the existing connection with the 2" vertical vent (about 4' away) through 3 wyes. I am not so sure about that given the convoluted path but seems to make sense? Thoughts?
I hope this is not too confusing, I can take additional pictures of course if needed.
PS: The last picture shows the (convoluted) path from shower drain to 2" vent, in yellow. Is this venting adequate?
PPS: Yes, the i-joist hacking is beyond ugly and scary ... I think the original plumbers got away with it because it's a double joist and the other one is untouched (except now for the 4" hole I added, but I know that's ok). I do plan to re-inforce it though with 3/4 plywood on side, and 2x.