attached photo - We are looking from the floor below the bathroom being remodeled. the new bathtub is over our head and I am looking to run the drain line.
(C) is the existing p-trap for the shower with (A) being the vent up and drain down.
Two plumbers have told me they will tie the bathtub drain into the existing shower drain going through joist at (E) and tieing in behind the joist (D) and use existing vent/drain (A).
Shouldn't I run the bathtub drain through joist at (E) and run to (B) with a vent that will tie into the vent (A) halfway up the second floor wall and 90 to (A) for the drain? 1/4" slope per foot
Can I tie into drain at (A) with fitting one on top of the other or do they make a fitting that will tie both into (A) at the same level?
I can not answer the venting questions you have but would like to address the framing.
I see only two nails on the "Joist" or "Blocking" piece to the left of 'B' in your picture. Have the floor joists switch location here? If they are floor joist and not blocking do they bear down on that wall that I assume the vent runs through?
Remember no holes should be drilled through your floor joist within 2" top or bottom and the max hole size in most code books for a 2"x10" floor joist is 2 3/8". That joist has a few large knots in it and I would be adding another new 2"x10" to it before I drilled a new hole.