Since your frame style is already very similar to 971’s, you might want to take a look at how they do the bumper mounts too. A sheet metal frame fits against the bumper backing with draw latches. The matching latch hook is riveted to the robot base. It’s a pretty secure fit, and they’re fairly easy to remove (just watch the clearances around the superstructure). We’ve done the bumpers as one piece like this for the past few seasons, but you could do them as halves, also. I’m not sure it would work well if you wanted each side to have its own bumper.
Considering that it’s a sealed, COTS transmission, I’m not sure how likely it is that you’ll actually need to remove the transmission for repair, so unless it’s easy to make it removable through the bottom and has no adverse effects on the overall strength/rigidity/manufacturability of the chassis, it might not be worth worrying about it, as long as there is a way to get them out. Since the transmission output is cantilevered over the top of the chassis rail, it has to rotate some to take it through the bottom without bringing it in towards the centerline.