Advice needed for plumbing: adding fixture

[moonchild]

Hi, I am add a tub a existing shower. All pipes are 2", except the tub p trap is 1.5".

So Currently(Top drawing), I have a Wye from the existing vented shower drain line, then I realize, the tub trap is a bit too far from the vent 6.5ft. Adding a new vent means I need more turns to route the drain under a wall to add the vent.

So I am thinking if it makes sense to change it to the bottom drawing: remove the Wye , and have the show&tub drain goes right under the wall with a 45degree. My question is

1. Do i need to add a vent? It is about 5.5ft from the vent, a bit more than the UPC's recommendation of 5ft , giving that IRC says 8ft for 2" pipe, will I get away with not adding a vent? Given the I can make sure the weir of of the both p trap is below the top of vent?
2. if I do add a vent, can I add between the two P trap (so i don' need to worry about the slope being accurate)? I learned that trap should be after the trap, in this case, the vent will be before the shower trap...though there is another vent after the shower trap (existing vent)
3. The wall 2 where the new vent will be inside, is only 5 inch from the shower trap outlet, if i can play with the p trap's angle and get it fit, the p trap arm goes to the drain will be very short, will there be any concern?

I also added the old plumbing to the top right of the drawing.

Thank you in advance!