1. a high flow valve is exactly how it sounds like; more gas is able to flow out of the mag which leads to an increase in fps. whether or not you asbsolutely need it dependes on you. do you want all your mags shooting at the same fps and similar gas consumption (more gas used = less mileage)? keep in mind that a high flow valve is not necessary to run stronger gas.
4. yes that is normal. the outer barrel will pivot to allow for the camber to clear out of the way and allow the slide to move fully back. the real guns do this too, i think it is called delayed blowback but i'm no expert in real steel firearms.
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