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1) Yes there are lots of li-ion batteries, just as with anything else in life. You pick the one you want based on size or capacity.

2) Only protected ones, plenty of unprotected ones are 18x65, and 90% of batteries are unprotected so just buy the right ones.

3) Very few flashlights require proprietary batteries. If they don’t there’s no reason you can’t use other cells that aren’t from that same brand. Just do some research and find out what the best ones are.

4) That’s because manufacturers have been pushing lumens higher and higher. If you want something that lasts longer then just use high or medium and not turbo, or get a flashlight with lower turbo output.

5) Obviously a huge LED that produces tons of lumens uses lots of power. They are still far more efficient than previous LEDs.

6) Very few flashlights use special battery packs, those are usually just super high output flashlights that cost like half a grand. If you don’t want a proprietary battery pack then simply don’t buy those flashlights.