OK, I'll chime in...batteries can do what called "sulfating"...where a build up starts to form on the anodes of one or more cells. This effect sometimes reduces output current and increases charge voltage. That COULD be why you're getting 8 volts...only way to tell is swap out the battery unless you have an auto shop quality tester.
I think that the light deal may have been a fluke. Did ya check the voltage at the light socket to see? Analog meter is better for this cause it'll show a momentary jump in volts due to a flakey switch...a digital meter won't.
It'd take alot more than 8volts to instantly blow both bulbs. 8 volts would just make them nice and bright. Have to be more like 12volts.
John gave you alot of good advise too. This is the stuff that takes alot of patience when working on the old iron...