Double stacking 18350s in a Mech Mod

[dr g]

I don't see what that matters.

Few people use any sort of protection in their mech mods other than simply using safer chemistry batteries. If something were to occur that would short out and cause venting with stacked batteries, it would also happen with a single battery. You hardshort a battery long enough it will vent, it doesn't matter if there is one cell or a million. The key is not putting your batteries in that situation to begin with

While I am with you about the "rules" thing and people not understanding actual dangers, fact is there are several failure modes possible with stacked cells that simply are not with single cells or fixed, protected battery packs. Whether these are consequential to a person is an open question. At the very least, stacking requires a modicum of caution and attention above and beyond single cells.

I would not stack or run batteries in series for PV use. I find single cell perfectly capable of doing anything I need (including vaping up to ~8v)