Here's the scenario. The house is a 2 bedroom 1 bath ranch with a massive add-a-level additon. The addition extends out the back (kitchen) on the main floor and the second story covers the entire main floor. The completed house nealy triples the house size. The new upstairs contains 3 bedrooms. A full bath is between two bedrooms. A master suite includes a master bath with 2 sinks, a drop in whirlpool tub, toilet and shower. The pictures and questions pertain to this bath group. A laundry group is present on the upper floor also.
The original plumbing includes a 3" sewer main and 3" wet vent stack serving the main floor bath. This stack is not being used for 2nd story waste but rather a wye connection is present in the basement which connects the new soil stack to the existing one under the lowest main floor bath group connection via a wye. The existing wet vent stack will run up through the upper floor then connect to all other vents in the attic before exiting the roof.
The wyes shown in the pictures accepting long bend 90s are for the master sinks (left most) and the laundry tub and floor dain (right most). Now for question #1. Is it acceptable to enter the tub drain through a reducing bushing in the straight end of the combo wye? Iwas concerned whether this causes solid waste buildup where the 1.5" line enters the straight inlet at the 3" combo wye. Question #2, can the upstream 2" toilet vent serve as the tub vent or will i need to install an additional 1.5" vent off a san tee on the tub trap waste arm before it enters the wye?
Question #3, is there a preferred order of connecting the toilet, shower and toilet vent to the 3" waste line that runs into the afforementioned combo wye? Currently the order will be toilet at end then down the closet bend 2 feet where a reducing wye connetcts the 2" toilet vent (raising 45+ degrees then vertical to attic), then a flat wye receiving the shower waste and finally (providing no additional vent is needed) connection to the curved end of the combo wye.
Other notes: there is adequate slope on the waste lines with some notching of the bottom of the floor joists, not exceeding 1/6 depth. The add a level is constructed so that there is a 5" chase between the bottom of the new upper floor joists and the finished main floor ceilings (1.5 separator plate plus existing 3.5 ceiling joists). The combo wye is not glued yet.
Alternately I can send the waste line closer to the wall and insert a reducing wye and long bend 90 in place of the combo if that is a cleaner way to plumb it.
Thanks in advance.