Well the brochures said it was to save weight.  They shifted from the beefier 5 bolt gearcase to the lighter smaller weaker design formerly used on the 7.5’s.  It was most likely done to save weight and cost.  Not sure what year that happened;  might have been 1958.

The difference has been noticed by 10HP Owners.  Maintenance is vital on the newer lighter gearcase;  little tolerance for water intrusion or inadequate lower unit oil.

Those gearcase dowel pins aren’t actually missing, they just got pressed flush into the casting when someone (previous owner) misaligned the hole in the bearing and assembled the gearcase.

That impeller housing “spacer” helps seal the impeller housing by holding a certain amount of water in that tube when running, and keeping exhaust pressure away from this area.  Once the spacer/orings are in place, you will notice water spilling out of the exhaust housing around the driveshaft tube near the gearcase when the engine is running, this is perfectly normal.

Hard to advise here.  Perhaps posting a picture or two might help out.   There should be either a hole in the forward gear bushing where the pin fits into.  Sometimes, when the units are misassembled, the pin puts a “dent/dimple” in the forward gear bushing/bearing, so you should check the fit of the bushing/bearing over the gear to make sure it rolls freely.