Time for more battery education

[kronological]

I asked many of the same questions in this thread:

CR123A batteries...what's so great about 'em?

I understand the better brands have about a 10 year shelf life…is that their only claim to fame? In flashlights that only take one CR123A…can you use a RCR123A or 16340 instead? Is the RCR123A and 16340 the same? For all 18650 single cell lights, can you use 2 CR123As? Confused and don’t wanna get blowed-up…