As for holding the drive shaft in place that bolt has nothing to do with it. On the drive shaft on the chain case side there is a permanant bearing guide fixed to the shaft that turns with the shaft. This fit into the bearing housing that is fixed to the chain case side bulkhead piece with three bolts. The shaft cannot move laterally towards the chain case unless that housing were to fail and the bolts gave out.
That bolt has everything to do with holding the driveshaft in place.It performs the same function as the driveshaft nut on the non reverse chaincase, which is to draw the driveshaft to the right until the seal collar butts against the chain case bearing. You are right that the collar prevents the driveshaft from moving laterally into the chain case, but it is the bolt that prevents the driveshaft from pulling OUT of the chain case. With no bolt in place (or a broken bolt) the driveshaft is able to move laterally towards the clutch side. The only thing that limits this movement when everything is assembled is the setscrews on the clutch side bearing. If those screws come loose the shaft is able to wander from right to left about 3/16 of an inch. That is approximately the distance from where the screws usually bite into the shaft to where the shaft transitions from round to hex.