I can't answer your second question, but as to the first...
The plastic mini-mag lens is just junk. It's generally scratched when you buy the light, and only gets worse with time. The glass lens offers better clarity and the reduced chance of scratching. Highly recommended.
I can't quote figures for the mineral or saphire lenses, but I did do some measurments on the MagLight OEM lenses and some UCL lenses. The OEM lenses have a transmission factor of around 90% and the UCL lens has a transmission factor of about 99%. In other words, you can get a 10% increase in output simply by changeing to a UCL lens! Easy and cheap mod!
I didn't feel like writing (gotta get the Post Office before it closes), so I found this out on the Internet:
How different materials compare:
Plexiglass, as you would expect, is the least expensive. It is also the least likely to shatter and the most likely to become scratched. Mineral glass, even though it has been hardened by a tempering process, is more likely to break than plexiglass. But it is also more scratch-resistant than that material. Synthetic sapphire is the most expensive glass crystal material and the most scratch resistant. Because it is so hard, it is also brittle, and shatters more easily than mineral glass or plexiglass.
It is a very hard, transparent material made of crystallizing aluminum oxide at very high temperatures. Chemically, synthetic sapphire is the same as the natural sapphire used in jewelry, but without the coloring agents that give the gemstone its various hues.
When it is heated, the synthetic sapphire forms round masses that are sliced into pieces with diamond-coated saws. These disks are then ground and polished into watch crystals. (One reason sapphire crystals are relatively expensive is that the tools required to make them are costly.)
Sapphire (whether natural or synthetic) is one of the hardest substances on earth. It measures 9 on the Mohs scale, which is a system for rating the relative hardness of various materials. (Diamond measures 10, the highest rating.) Watch crystals made of synthetic sapphire are often marketed as "scratch resistant", meaning they are very difficult - but not impossible - to scratch. Diamond can scratch them; so can man-made materials that incorporate silicon carbide, which, with a Mohs rating of between 9 and 10, is, like diamond, harder than sapphire. These materials are sometimes used to make simulated-stone surfaces for furniture or walls. The watch wearer should note that accidentally scraping a sapphire crystal against such a surface could cause a scratch.
As far as I can see from data I've found, the light transmittance of sapphire should be very close to a high quality mineral glass at around 95%.