I've read quite a few posts and the rule that I am having the most difficulty adhering to is the:

"weir must not be above the trap opening"

of course, I will adhere to it but it is a pain. I am in Ontario so, assume international plumbing code (until I find out otherwise)

--- Now, is the reason for this because you don't want vent air to be trapped inside the fixture arm?

--- Does the WC have to adhere to this rule as well?
It seems rather impossible as the trap is inside the toilet.

--- If I have a main 4" vertical stack in my bathroom wall tying to the main waste out, can I stack 4" Sani-Tees (4"->2" on top for sink, 4"->2" for tub, 4"->3" for WC on bottom) ?

--- Will the vertical stack suffice as a vent for all three of these? (it is vented through the roof)

--- If this looks good thus far, can the main waste be tied to the WC using a 4"->3" Wye instead of a Sani-Tee + 1/8 + horizontal run to under WC + vertical to flange? I saw this posted somewhere and was wondering if there is only one official way to do this.

Any questions, let me know or point me to relevant posts.
Thanks for any help!