Why is the collision affecting your root bone at all?
set the root bone to kinematic and the character should be able to just get back up where it fell. You have to set pelvis to “free” on all angular and linear limits.
aside from that, just script it.
Align yourself to the other character all the time with a look at node on rotation.
that way you can have the game be 3d instead of faking 2d.
you can still lock the camera and it basically becomes a 2.5d allowing for side steps.