Joe,
I dont know why the storage voltage matterts so much either, It may be a safety thing as a fully charged pack has more energy to disperse in a failure than a partially discharged pack (i.e. more fire and smoke). I can tell you that it has been my experience that the batteries for my micro heli and plane have not gone bad as quickly since I began storing them properly.
Regarding your charging jar, that will probably work well to contain the heat and fire but I wouldnt put the lid on tight for fear it could explode in a battery failure. As you witnessed the packs put off a lot of hissing and smoke and can create a good bit of pressure in an enclosed space. I would hate for the jar to explode and pepper you with glass. I never charge batteries in my house as I would hate to have all of the smoke inside if there was as failure.
-10 C wow, that is COLD!!! I would think that you would have gotten a much better show if they had been room temperature. I know a cold LIPO pack does not perform nearly as well as a warm one. -10 C I am amazed, I never felt anything near that cold.
Here a couple of links to some fairly useful pages discussing battery care. I see thes pages referenced quite a bit on the airplane forums.
http://www.hangtimes.com/redsbatteryclinic.html
https://sites.google.com/site/tjingu...po-terminology