Reacting Propeller Torque?

Yes it has to react.

Say your engine can generate a maximum of 150 Lb-Ft torque. And you attach to the ground or back of a boat by a bracket which has 2 bolts on 16 (1.33 Ft) inches apart.

your bracket has to be able to resist the:

150 Lb- Ft/1.33 Ft = 112.78 Lb,

pull out tension on one bolt and push in compression on the other one.

Yes, it does.

Imagine you block the propeller entirely, say, fuse it to the casing, or tangle it in a huge wad of fishing net that causes huge drag (or instead, just fuse the two gears together, so they can't turn - oh, say, put a wrench between them.) In this case the motor, through the shaft and its bevel gear will try to spin the entire assembly of the propeller, its shaft, its two bearings, the lower bevel gear and the entire casing, making the whole thing turn around the motor shaft.

Ground2 is what prevents that and causes the torque to be transferred to spinning the propeller as opposed to spinning the entire assembly. It doesn't 'feel' much torque

the gears and the propeller. The upper two bearings are negligible, as they only transfer as much torque as their internal friction. But any torque put by environment on the propeller will be applied to the gear, and transferred to ground2 through the two lower bearings.

There will be some extra torque from friction of two upper bearings and the bevel gears, but that's just losses, as opposed to torque from the work done. I believe the torque will distribute equally between the two grounds, assuming ground 2 surrounds the casing instead of just being attached to one edge.