Yes, retrofitting a ground is fine if you're not extending the circuit to a new location. Take care that the new ground wire does not contact the existing neutral, you might want to "make it an insulated wire" with some shrink tube to sleeve it.
SE cable has a bare neutral, that is how it is made. Read the Chapter 3 rules for it, you have the right to re-designate it as a ground. But no obligation.
You cannot retrofit a neutral.
#10 copper ground suffices for conductors with capacity up to 60A.
Yes, the ground can come off any junction box that has #10 Cu ground going back to the same panel the circuit comes out of. Bare copper is fine.
You could use #8Al ground but would be difficult to splice to the water heater ground, and is an odd duck you might not find easily.
The receptacle must be rated for 75C thermal to get 50A out of #6Al. Also it must be rated for Al wire. Most are, but make sure.
There are two tricks to working safely with aluminum.
- Using terminals properly rated for aluminum is the first one (glares at UL and their blunder in the 1960's, too hastily rating 15A receptacles for aluminum without proper testing).
- Torquing connections to spec. (this science was only discovered quite recently, and on copper connections - but we can safely assume it is important on aluminum too).
The cheapest way to get a torque screwdriver is to get a 1/4” drive "beam type" torque wrench. Either use Allen sockets, or "drag link" for flat-blade, or a "bit holder" adapter for 1/4" hex screwdriver bits.