: : As for the vent, i figure i can just run it vertically up to the ceiling and connect it back to the main stack since it's the top-most floor anyway.

: : Also, How do actually access the drain pipes underneat the subfloor? I ask this question because I get the feeling that I can't just freely saw through the subfloors from the top as there are supporting nails that hold up all the toilet/tub/sink fixtures in the other side of the wall. If I saw through that subfloor, it would have no support.

: : On the other hand, I also don't want to access it from the floor below since doing so will destroy the kitchen-work below.
: : Any suggestions would be greatly appreciated...

You are talking about water piping and then you jump to the toilet drain piping. You can do the water. Drains are more of a challenge. You cannot tie one toilet drain into another toilet drain without a vent.
You can run your drainage by taking up the floor or from the ceiling below. I prefer the ceiling if I need to drop down below the joists but I usually do it from the floor because that room is getting remodeled anyway. I don't understand your explanation of why removing one subfloor would affect another. LonnythePlumber