1.) I didn't see HP mentioned once, so I assume that it stops increasing as well.
-How does playing without increasing Hitpoints *feel* to you players?
-What about you DMs?
Like kevin_video said, there is always the Toughnes feat, giving any 6th level character 6 hp. If you still have a bad feeling about this, E8 might be an alternative for your game.
As for the feeling, I can only speak from a GM's perspective. First, its a weird thing, and you need to get used to the fact that a large portion of the Bestiaiy can kill your players easily. But after it sinks in, you realize how much this can improve the game. After one or two tough fights, your players will realize that they can't affort to charge into any dungeon they find like brainless maniacs. They need a solid strategy, they need preparation, and above all, caution. They will start to take the world around them more serious. If your players are anything like mine, they might also surprise your with a display of cunning and cruelty you never saw before. Survival can be a strong motivator
This doesn't mean that I am f***ing around with my players or constantly putting them in danger of being killed. I run a fair game, where there are always several ways to approach a problem. I just made it clear that I won't cheat, and that there might be villains they cannot beat without preparing accordingly for the job.
2.) Is it a much higher casualty rate when you hit CR10 encounters?
I'd say that really depends on your player's knowledge of the rules, and how their characters are built. If they are accustomed to "normal" Pathfinder games, it may cost them a character or two before they attune to E6's reality. Do the character complement each other during combat? Are all players aware of the fact that the game works outside the standard mechanics for level-appropriate encounters, and therefore running also is an option to end a fight?
3.) Can anyone recommend some expanded spell selections to use specifically with e6? I already have everything from Rite Publishing and Necromancers of The Northwest.
I'm familiar with one or two Rite Publishing materials, their 101 spells series might be all you ever need. Personally, I am using an assortment of extra spells from the D&D 3.5 Complete series, Eberron supplements (we are playing an Eberron game), and some spells from various d20 supplements I found interesting. However, be careful with the amount of extra spells in the game, as you might end up creating some of the very problems E6 tried to avoid in the first place.