I'm wary of sticking that kind of stuff in my camelbak: they're pretty much impossible to clean properly, and you could easily end up with a ruined bladder
No, it's easy. Get the camelbak cleaning kit or just soak in milton when it gets manky.
Re the science - if you lose some salts due to sweat, then it stands to reason you'll need to replace it. Afaik you need the salt in your body to hold onto the water - more salt (ie from salty food) means you need more water to balance out salt conentrations, which is why you get thirsty after eating lots of salt (and you get heavier too).
The debate about sports drinks is, afaik again, about the 'isotonic' ones. These have the same salt concentration as a normal human body. But if you do the maths, isotonic drinks replace more salt than you need I think resulting in you not having enough water for the salt you have, and being dehydrated.
I think you do need SOME salt(s) though. quite a lot is excreted, so why wouldn't you need to replace it?
As for evolution - I'm sure half a million years ago early man was sitting around under trees in the heat of the day (along with most of his prey) thinking 'bugger this, I'll hunt this evening'. And it probably didn't take him 4 hours of continuous exertion to find a meal either.