Yes, lithium batteries have a 10 year shelf life. No, I would not suggest leaving the batteries in the light for 10 years while not using it. Some lights have parasitic circuits and some simply drain the batteries over time while not in use. Large maglites are well known for this problem and that is why the batteries end up leaking. Lithiums don't leak, they vent. If these batteries are drained too much or one drained more than the other, when the one goes dead, the other will sort of try to charge it and they can explode. If you leave lithiums in a light for a long period of time, check on it every few months at least. Open the battery tube and let the batteries air out. 123As weren't originally designed to be used in sealed flashlights. They vent to release pressure in the cell to keep them from exploding. Opening the lights every once in a while will allow a place for the pressure to go and prevent your light from eventually exploding. Storing the batteries in most lights for months is probably okay, but I wouldn't suggest doing it for years. Single cell lights will be safer to store a battery in as you don't have two or more cells to charge each other and cause an explosion. I think Fenix clicky lights are also safer for this. If pressure builds inside a Fenix light, the tailcap switch boot pops off letting the pressure get out of the light (you may have to find that part later though).
Hooked on Fenix-
There are so many things wrong with the info in your post that I don't know where to start, so I'll just take it from the top.
1. Maglites don't drain cells when they are turned off. There are a large number of people who have problems with leaking cells in Maglites because of two things; lots of people have Maglites, and lots of them use alkaline cells which are inherently more likely to leak. When a Maglite is off, no current is drawn from the cells.
2. A "dead" cell in that is discharged in series with a "good" cell does not get charged by the good cell, it gets overdischarged. This can cause problems, but using quality primary cells mitigates the chances of that to a large extent.
3. Lithium cells certainly can leak, but the electrolyte used is volatile and evaporates quickly. Venting is the same as leaking. The cells contain what is the equivalent of a burst panel molded into their plastic gaskets. When the panel ruptures, the cell has vented and leaked. This is not something that occurs on a normal basis to "release pressure in the cell to keep them from exploding."
4. Single cell lights are no safer for cell storage than multiple cell lights. Storage implies that the light is OFF, so there is no "charging" (Incorrect term, see point #2) going on in multiple cell lights when they are sitting unused.
Black Rose-
The smell from CR123 cells isn't typically due to venting. It's more from the slow loss of volatile compounds through the gasket material. The materials used are close to, but not completely, impermeable to the electrolyte used in the cells, so over time a small amount of electrolyte will escape without the vent panel rupturing. When stored in a closed container (a flashlight), this small amount of electrolyte vapor is concentrated such that there is enough that you can smell it upon opening the light.
highseas-
Don't worry about it. Go ahead and store the cells in the tube. As long as there is no path for current to flow and short the cells, they should be fine.
