I so far understand the basic idea that breeding lessers, mojaves, het russos, butters, phantoms, mochas to each other gives you a 25% chance of a BEL. And do ALL of these when bred to another of their own always have the same chance of making a BEL too?

Chance are the same yes. but not all will make a BEL
lesser/mojave/het russo/butter/phantoms/mocha/mystic bred to a lesser/mojave/het russo/butter/mocha will make a BEL.

phantom x phantom makes super phantom
mystic x mystic makes super mystic (guess they look like super phantoms tho I can't really recall seeing a picture)

What are specials, and mystics? How do they work? What do they look like?

already explained.

Where did the lesser platinum come from? I remember reading it came from platinums or something...? Or does it just resemble them?

listen to Russ Lawson, genetics are pretty simple, the "hidden" gene is part of the BEL complex, when combined with the lesser gene it makes a platinum, problem is the "hidden" gene doesn't appear to be visual on its own, but if you bred a platinum to a normal, you know all the normal looking babies are "hidden" gene snakes.

What is it I hear about phantoms and mystics being the same thing?

just like butters and lessers they are the same thing. different lines of the same morph. they still make the same "supers", combos with other morphs behave the same. just like a lemon pastel and a bell pastel. same morph diffferent line.

Are there any other BEL-making mutations I have missed?

not to my knowledge