To answer your questions, the moon itself is subject to the sunny F16 rule. If you want to make a shot of just the moon itself with 400 speed film and have the ability to spot meter as you indicated, I would meter the moon only at the reading indicated for the 400 speed film and give 2 -3 stops more exposure. This would give you an exposure that would render detail and the value representive of what the moon would be. So, typically this would be 1/125 second at F16 in the use of 400 speed film. For 100 speed film this would be typically 1/25 second at F16.

For your second question since you are using a double exposure the exposed value of the sky of the moon only exposure as well as the second exposure will be additive. So you would be best served in shooting a double exposure in a situation where the light contamination from external sources is limited so much as is possible. In the case of a double exposure one would normally reduce the exposure of the second exposure by one stop since the film would have broken exposure threshold with the first exposure.

I hope that this gives you some direction. Good luck and have fun.

Donald Miller