Well yes you could add all that complexity and do it. Or... you could pop them in a dedicated charger, pop some freshly charged cells on the mod and be on your way.
I tend to avoid unnecessary complexity. Less to go wrong.
Maybe. Specially with single battery mods with slower charging and the only way to get going again is to be prepared with some fresh ones.
My guess is that a good 2x ~3000mAH will last most people a few days and almost everyone at least a day. USB chargers are everywhere already, and with a charge rate of ~1A, it shouldn't take much time to get operable even from a depleted pack. With a bit of engineering (we gotta do something, right?), we can avoid the unnecessary complexity of being bound to special external chargers and continuously opening/closing your vape + mechanical contact stresses, battery sleeve wear and tear..
If we're looking for a minimal complexity circuit, we can try think up some under-voltage + fuse circuit, like some zener + mosfet + polyfuse set up.. imho this should be part of a bare minimum DIY scheme if not using a BMS.