Adding 2 Piece Powder Room + Laundry
All and all it looks good!

Regarding the Powder Room:

It is totally fine to join the powder room and laundry room on the same branch (as in your last picture). In addition, if the height is limited, instead of using a 3x2 sanitary tee on the vertical pipe to the toilet, you can use a 3x2 wye on the horizontal pipe before the toilet 90° to connect the sink and still vent the toilet.

Back to Back Sinks:

Yes, you can install sinks back to back. The fitting used here is a double sanitary tee or sometimes referred to as a sanitary cross.

Additional Comments

Note that some locations require that there is a fire-rated separation between the garage and the rest of the house. You can still penetrate the wall but you may be required to provide an approved fire stopping around the penetration. What you use will depend on how the wall is constructed and the drainage materials used. The same would apply to the water pipe penetrations.

Where I live we cannot use 90° elbows when transitioning from vertical to horizontal. We are required to use two 45° elbows. It's a bit silly, as we can use a hub x hub 45° and a fitting x hub 45° together and get close to the same radius as a short sweep 90°, but code is code. I know that many other jurisdictions/codes allow the use of long sweep 90° elbows for vertical to horizontal transitions.