Long Ride Hydration Alternatives

I'll carry to bottles on a rear cages behind my seat and one bottle on my aero bars. At about three hours I plan a stop at a gas station and purchase bottled water which i mix with my nutrition. three bottles, one stop equals 6 hours for me.

I use two 24oz bottles on the frame and another in my back jersey pocket.  That usually lasts a little over 2 hours depending on conditions.  I don't think I could carry enough water in anything for a 6 hour ride without stopping somewhere.

I can go 5 hours without stops with this setup: in 90F ish weather.  Podiumquest (between the aero bars 28 oz chamber for Infinit, 16 oz for water, 1 downtube 24 oz (you have an extra cage so thats an extra 24oz for you) and then a neverreach for the back 64 oz.  So that is a total of 28+16+24+64=   132 oz for me 156 oz for you with the 2nd frame cage. 

With 140 ish oz on the fame the bike handles a bit like a tank and you feel the hills.  If I needed more than this I would use a fannypack camelback (64 oz).  If that was not enough then I would go with the 100 oz backpack camelback.  I think though that I will do 2 smaller loops of 50 -60 miles and replenish rather than care more than  130 oz on the frame or go beyond that with a camelback. 

1 aero bottle ~ 28 oz
1 bottle on frame 24oz
1 bottle in jersey pocket 24 oz

If I stop at a gas station it's usually for a potty break, and I just fill up the bottle in my jersey at the sink.

I don't have much experience with rides that are longer than a couple hours, with no recourse to gas stations.  That being said, I'd probably prefer a Camelbak over rear cages or a fuel belt.  I ride on roads whose pavement seems to have been applied with a trowel, by drug-addled monkeys -- if I'm going more than 15mph and hit a rough patch, things just fly off my person and the back of my bike (I've had granola bars and my ID fly out of my pockets, and the rear taillight has been ejected off the back of my bike a few times.)  I feel like adding more things onto myself or the back of the bike is just asking for MORE stuff to fly off and have to be re-gathered; at least a Camelbak is securely attached to your person.  Plus, you don't have to reach around and grab it, then bike one-handed -- you can just sip it through the little straw, and then you feel like an astronaut or something.

As for your side-note question It depends on how scuzzy the gas station is.  If it's really clean and nice, I'll use the water fountain or some other water tap.  If it looks like I might get syphilis from touching a surface, I buy a bottle of water.  I prefer to use the taps whenever possible, for environmental reasons, but I'm kind of squeamish about gas stations sometimes.