1 - Length. An 18650 = 2xCR123 cells side by side. 18500 = length of one and a half CR123 cells side by side. Two 18500 = 3xCR123 cells.
2 - No. A light that takes 2xCR123 cells will accomidate one 17670 cell. If you want to use an 18650 cell, the 2xCR123 light has to be bored out to accomidate it. Your stock G2 will easily take a 17670. But an 18650 will not fit.
3 - Best charger, without a doubt, PILA IBC model. Lighthound.com has started selling it. One of the best online shops out there. The charger isn't cheap, but it is the best. And no, there is no nearly as good version for a dirt cheap price. There are lower priced chargers that demand constant attention while charging cells. I have an Ultrafire WF-139 charger. It's as basic as it gets. Gets the job done, but requires keeping a constant eye on it. Avoid the Trustfire version of this charger. Looks just like the Ultrafire, but is extremely dangerous to use; even if you keep an eye on it.
4 - As mentioned above 3xCR123 lights will accomidate 2x18500. But once again, only if bored. Stock bodies will work with a pair of 17500 cells.
To be honest, I'm not too heavy into rechargeables in my inca. lights. Others are more knowledgeable. Let's just say it's extremely important to learn everything about such cells before you begin using them in your lights.
Hope this helped.