The snap ring was designed to hold the lower race in place during assembly. It's not necessarily needed because you can install the worm shaft with the race already in the box. I've had a couple boxes apart. Neither of them had the snap ring.
The tube can be removed. It is brazed to the disc on the bottom side. This disc is essentially pressed in to the bottom of box. If you remove it, you will need to tap it back in. Clean it first...and use some sealant.
Tim