I'm looking to do a renovation that would steal some space from my current upstairs bathroom to make room for a master ensuite. In order to accomplish my plan, I would need to relocate my main bathroom's toilet to the right, and I would need to add a toilet (for the ensuite) to the right of this.

Here is my current main bath layout:

Here is my new layout for the main bath and added ensuite:

Here's what some of the symbols mean in the drawing:

• The light-purple floor is the footprint of the current main bath
• Purple square in the wall represents the current toilet's lateral location
• Red square in the wall represents the approx lateral location of the powder room toilet on the main floor below
• Yellow rectangle above the wall represents the approx lateral location of a jut out of the basement wall (so I'm guessing the main sewage stack is somewhere within this?)

The problem is that my floor joists run top-to-bottom rather than left-to-right and you can't drill through joists for a toilet drain. I plan to install wall-mounted toilets which means that I would build out a false wall behid the toilets (about 6" deep) to contain the in-wall cisterns. Since I'm using wall-mount toilets, does this enable me to run the toilet drains ABOVE the floor joists within the 6" false wall? I was thinking something like this:

With that said, I suspect that the main sewage stack might actually be located somewhere between the two proposed toilet locations (i.e. somewhere within that yellow box in the drawing). I suspect this might be the case because:

• The closet on the main floor has a dropped ceiling and is directly below the main bathroom toilet. I suspect the main bath toilet is draining left-to-right within the closet's dropped ceiling toward the Powder Room toilet
• There's an awkward jut out in the basement that is likely to conceal a pipe of some sort (and the jut out al

If the stack happens to be location between my two proposed toilet locations, would that make it easier for me to build my proposed changes (since the main bath and ensuite would have a short distance to the stack)?

For reference, here are the floorplans for my house:

So to summarize my questions:

1.) Can I run the toilet drain for a wall-mount toilet ABOVE the floor joists?

2.) Would my plumbing situation be easier if the main seware stack is location between my two proposed toilet locations?

3.) Could I run into capacity problems by attempting to add the ensuite bath (i.e. maybe the existing plumbing can only handle one bathroom upstairs)?

4.) Are there any other plumbing concerns I might be overlooking?

FYI, I have no plumbing experience and just started reading about this stuff a couple of days ago :) Thanks for any help!!