1) Can I use an MTB HTII bottom bracket with a (10spd) 105 crank? The current one on my winter/commuter/tourer bike is knacked and I thought that something with better sealing would be, er, better.

2) Are CO2 cartridges a one-hit job? Used one for the first time the other day after a puncture (road tyre) and when I made sure it was empty before I threw it away there seemed to be quite a bit left in it, possibly enough to have done another tube in the future

1 yes. But the centre sleeve will be too long and i think the cups are slightly thicker. It’ll all work though. The sealing isn’t hugely different between road and mtb.
2 you can keep them. But they do slowly drain/leak. And there probably isn’t as much co2 in there as you think. A 700×25 to 100 psi pretty much almost exactly empties a 16g cartridge.