The front brakes do most of the work. It is not the case, as you assume, that both front and rear contribute equally to braking. If you didn't use the front brakes, or even if you tried to have them apply the same force, the rear wheels would absolutely skid.
If you want an extreme visual example, check out stunt cyclists who brake so hard the rear wheel comes off the ground. Pretty easy to see that the rear wheel isn't contributing to braking when it's in the air.
Bingo. When I was a kid, I once flipped my bike because I was only braking with the front. I learned not to do that, but to an extreme — I then braked almost always with just the back.
It wasn't until I got older and faster that I realized that braking just in the back is not as effective, since the braking action makes your nose dive and puts more weight over your front tire. I now brake with both sides, but always first in the back.
My commuter bike only has a front brake. You can shift body position to make it pretty much impossible to go OTB. Meanwhile my DJ bike only has a rear brake, which at best "strongly suggests" you slow down.
In cars that only have rear motors, unless you apply the brakes (or they have some blended braking setup) then ALL regen is via the rear motor.
Obviously, front brakes are capable of more braking, but the limit at the end of the day is the regen of the battery itself and how much the inverter can handle regen. RWD-only EVs are already capable of ~75kW of regen. Moving to FWD to try to gain regen is only marginally going to give you better efficiency, if anything. It only matters for harder braking.
If you want an extreme visual example, check out stunt cyclists who brake so hard the rear wheel comes off the ground. Pretty easy to see that the rear wheel isn't contributing to braking when it's in the air.