The drain line follows the plane of the trap and goes through the joist at the top of the photo - the angle of the photo makes it hard to notice.
The "2 90s" to which you refer are in fact a T and a 45 for the vent which goes off to the left in the photo.
The only clear bit of copper pipe that I can cut into is the riser from the trap. But I think that'll work because I have enough height, as I explained before.
The toilet line worries me more because the coupling and new ABS bits would move the flange over by 3/4". The new narrower toilets will have no problem fitting in the space available: What does concern me is whether or not folks would notice that the toilet is a little closer to one wall than the other. Is an offset flange, then, a sensible idea, or does it present its own set of issues (greater risk of blockage?), or is it just not worth bothering about?