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1) Yes, you can charge two 12V batteries in series, then use them in series
to get 24 Volts, and tap off the center between the two batteries to get
12Volts for the automotive accessories. However, I think the voltage
choices are 28 Volts and 12 Volts. You can buy a DC-DC converter and get
12Volts at about 10 Amps from a 28Volt supply. This should be enough to
power all your fun accessories, if not, get two convertors. I am going to
use a 12 Volt system. I can not imagine overloading the alternator, using
more than 40 Amps continuos current in such a small plane.

2) Capacitors will dampen the voltage spike, but they will cause a huge
inrush current when you actuate the solenoid. You will have to supply
current to turn on the solenoid plus charge the capacitor. This will put a
big load on your switch, and wear the contacts out faster. The best
solution is to add a flyback diode. This is a diode in parallel with the
solenoid. The cathode should be on the positive side. If you still get
noise, you can completely isolate the solenoid by putting another diode in
series with the paralleled solenoid & flyback diode circuit. Keep all the
diodes as close to the solenoid as possible. I drew the following schematic
in ASCII. If the spacing comes out wrong, cut and paste this into Notepad
to see it better.

POS--->I---|--solenoid--|-----GND
           |-----I<-----|

Ed
Watsonville CA