Upstairs Bathroom Drainage Questions

I'm adding a bathroom upstairs in my 1927 home in Ballard, Wa. The bath is going into the center of the house with a dormer and my stack goes up the side of the house. So, I plan to send a drain staight down and connect to one of the horizontal drains that goes from one end of the house into the stack at the other end. I plan to have a bathtub, toilet, separate shower and 2 sinks up there. I tried to attach a jpg of the layout. The dotted lines are floor joists.

So the questions are:

1) What size drain should I run down through the wall? I was planning a 3 inch pipe. Is that big enough for a toilet drain?
2) I'm restricted a bit by floor joists, but if I have my toilet go through the joist (draining forward rather than backwards) I can create a main drain pipe where the tub, toilet and shower all attach in run parallel to the joists and exit left down through a wall below. Is this OK? 3 inch pipe?
3) Venting seems like it would be a trick, since the whole thing would be in the floor. I could vent it on the one side of the main drain where the tub is, and then right above where the drain goes down...

3" is fine for the toilet. The toilet will need at least a 2" vent.

The two lavs can come up with a 2x1.5x1.5x1.5 fixture cross in the middle, using a 1.5" vent. Most plumbers use 1.5" traps on these and use the provided reducer to 1.25" for the tailpiece.

The shower needs a 2" drain and p-trap, and you would use a 1.5" vent for it.

The tub would use a 2" drain, the trap arm could be 1.5" and a 1.5" vent.

The vents can be tied together at 6" above the lav height.

If you use a fixture cross for the lavs, that gives you 4 vents.

layout

Conditions would dictate how you run the pipes, but unless you have very deep joists you would not be permitted to, nor would you want to, run the pipe through the joists. Normally the best way for something like that is to run all the pipes parallel to the joists to an intersecting wall and then drop down and join them together in the basement, under the slab, or in a boxed chase, whichever works best for your building.

Vent where?

I spoke to a friend of mine who does a lot of plumbing. He said that I could just run a big 3 inch pipe the length of the room, parallel with the joists, vent it with a 2 inch vent at one end and with a 2 inch vent over where it goes down (at the other end) Then, I can run the tub, toilet and shower into that one main pipe. Of course the lavs would have their own vent. So three vents, one in the wall with the lavs, one in the wall next to the tub, and one in the wall next to the shower, essentially the big 3 inch pipe is a wet vent. Does that make sense to you guys?

Thanks a lot, by the way.

I guess you're not getting this inspected in Seattle.

I'm sure your friend never has his work inspecter either.
I feel for the poor people that buy the home from you.