For the most part, it is all inane urban myth.
> I read that even the BPA free variety of plastic can leech contaminants if warmed
And I read that some people have been abducted by aliens.
Problems with the statement (about plastics, that is):
WHICH plastic???? There are tens of thousands of utterly different sorts.
Most if not all food grade plastics do not contain 'contaminants' of any sort. PET, which is used for all sorts of bottles, is just PET - no 'contaminants' at all. Freezer bags are polyethylene: just polyethylene, no contaminants.
There was a scare about BPA in Lexan, but the risk was really only to the workers at the plant. But the scare sure boosted sales of other sorts of water bottles for a year or two as people rushed to replace their perfectly safe Lexan bottles.
> Is exposing a plastic bottle to water at temperatures just shy of boiling more
> dangerous than the average daily exposure to sun and heat on the side of my pack?
Yes and no. Boiling water may cause the plastic to melt down and lose shape. No danger to you, except of getting boiling water all over your hands.
> Should I keep my water bottles hidden from sunlight?
The sun light won't hurt the plastic, but it might cause fungal growth in the water.
> Should I change them out more regularly as I resupply in-town?
I do - about every 2 -3 years. They get scuffed. I like them shiny.
> Is one brand of bottled water better than another regarding their use of plastic?
Actually, most brands of bottled water are no better than the town supply - and sometimes measurably worse. The whole bottled water industry is one vast rip-off. CocaCola Amatil will fervently disagree of course - but they want your money.
> re my FBC bags slowly killing me?
Geez mate, I dunno. How many bags do you eat a day? I know they aren't good for the mid-ocean sea birds that swallow them.
Hope this helps