Original Electric Furnace

In my quest to a more energy efficient home, I realized that re-using what already existed, and has worked for 20+ years, might be a good idea. Also, this is good for carbon reduction ("Reuse"). So that means I need to assess what I already have, as far as HVAC. Our home is equipped with an electric furnace from Lennox, model E11Q5-941-1P with Honeywell control. Here is a picture of the whole furnace:

Here is a close up on the label:

And here, the electrical wiring schematic:

From the information I have gleaned so far, this is a switched system, meaning it doesn't have a sequencer like modern electric furnaces. Instead, relays are used. Maybe this is a good upgrade to make, installing a sequencer. The blower is a 5 speed blower, although only two seem to be used (unlike what the wiring schematic says): Low speed for startup, then high speed.

Here are the features I can so far understand from the wiring diagram:
5 heat strips, or "element-electric heat", element 1&2 on circuit breaker 1 (CB1), element 3&4 on circuit breaker 2 (CB2), and element 5 + blower motor on circuit breaker 3 (CB3).
Each element has its own limit switch, although they are all called S2 on the wiring diagram.
Switch K1 drives the blower motor. This is the switch that is turned on when the thermostat manual switch is set to run the fan. If K1 is OFF, and K2 or K3 is on, then the blower runs at low speed. If K1 is ON, then the blower runs at high speed.

I will have to research a bit more to understand how the thermostat drives the different switches. It looks like upon a call for heat, the thermostat will close K2, which will run the blower on low speed, and energize HE1, 2 & 5. K2 AUX contact will close, possibly after a delay (?), and energize K3, which will turn HE3 and HE4 on.
I don't fully understand how K2-AUX work, or how K1 is energized (aside from the manual switch).

Before I think of adding solar heat to the system, I must understand how this furnace works, and how I can upgrade it to run an auxiliary source of heat.