" To conserve space and to enhance the appearance of the toilet,"

"We can/will add a bump out behind the toilet to give us the sapce we need for the tank unit and to bring the drain more in line with the existing one, but I don't think I want to bump out a full 12"."

And how do these two work?

Select a toilet, open the PDF for that toilet and you will answer most of your questions.

A wall mount saves little space, because you still have the minimum required space for a WC compartment.

I'm sorry, maybe I didn't word my question properly or use the proper terminology. I know there is such a thing as an offset flange for regular toilets so that you have ability to adjust slightly for drain position. I'm wondering if you have any such flexibility with a wall mounted toilet - to angle the drain pipe from where it comes out of the toilet (above floor level) to where it would connect to the existing drain in the floor.

The wall mount toilet requires 2 x 6 walls and I have 2 x 4 walls so we either need an adapter or we would need to bump out the wall slightly to give us the depth of a 2 x 6 wall. The toilet then projects 21" from the wall, as opposed to 28-29" for a regular elongated toilet. So if we build the wall out 2", we save 7-8", for a net gain of about 6" - which can be a big deal in a small space. BUT, if we have to build the wall out far enough so that the drain drops straight down from the unit into the floor drain, we don't gain any space savings. I still might want to do that just because the wall mount is so much sleeker looking and easier to clean - no nooks and crannies.

I have looked at the pdf's for the toilet I am considering but they don't address options for installing drain pipe once it comes out of the WC unit.

There are not any options similar to what you looking for, because wall mount WC are a preplanned dedicated fixture.
A wall mount eliminates the floor flange entirely.

However, even though your fixtures "appear" to be stacked, the waste line could be offset in the wall causing another set of problems.

A visit to a terry love site/forum will afford you an opportunity to search for waste line drawings and layout.

Plumber will easily replace the regular toilet with a wall mount, I don't have any other suggestion all ways are given above.

Here is a link that might be useful: hiblow parts

"ElizabethCampos"

Is there a PDF detailing the process, other than a parts list for associated equip?