Mark -
In no certain order:
1) The timer is already set-up for a second rinse as you now know. I would have said before seeing your pictures that I suspect all that is needed is a couple more wires added to the harness to activate the motor run circuit, the wig-wag circuits, the mixing valve, etc. to take the second rinse from dead to functional. BUT, seeing the diagram, it looks now like all you would have to do is install an on/off throw switch in what is now the wire that tells the timer that the second rinse sw says "Off". In other words the machine is now permanently wired with the second rinse off. Just add an "on" switch and the rest should work, at least I'm thinking that.
2) Yes, there aren't usually spray rinses in the second rinse in this era machine. This was not a highly common BD option, so having a machine with a 2nd rinse capable timer without using that feature is somewhat unusual.
3) Pre-wash --- starting in the very late 1960s, Sears sold a great many machines with Pre-wash cycles. They always, at least in belt-drives, drain and a short spin prior to end of cycle. Not nearly as many models had an automatic advance option into the Normal cycle from Pre-Wash / Pre-Soak.
4) Agitator - Yes, this and the other large capacity agitator of the same period, the Penta-Vane, both abruptly end with a wide flat agitator cap. This is how the same transmission (from late 1973 onward) is used in both the large and standard capacity models.....the high water level is relatively MUCH higher up the agitator shaft in the large cap models.
To allow for agitator mounted softener dispensers, an extension is added via a different agitator bolt. This extension allows use of the same softener dispenser that was used in standard tub machines. I surely have a picture somewhere...
I hope this sheds some light.
Gordon