Check out this guide for any and all questions you may have. The length of your barrel and magazine is up for debate. Generally for such a low airflow blaster, try a 2" barrel of cpvc and maybe 4-5" of pvc or copper for the magazine. It also really depends on if you'll be using stock darts or stefans and even what kind of stefans at that point.
Just buy excess cpvc and or copper and experiment yourself with your blaster after you have your other mods finished.
Should work fine. Also remember that 2 magnets can push as well, so your catchspring can also push up from the handle if you're into that (I am).
Probably not better than using a regular spring in either case, but maybe you'll discover something interesting along the way.
This seems like a fantastic idea, as two magnets repelling each other would probably be able to make for a more secure catch than the tiny puny springs we use similar to click pens. Not to mention maybe allowing for a more firm trigger pull potentially as well.
A Recon spring would be perfect if you just want a tad more "oomph" to it. It's relatively weak enough to not warrant too many reinforcements even though it might be a good idea to anyway. However, you probably won't get a good range increase if you don't remove the AR mind you.
Also, as far as the priming rod goes; just grab your fav claw hammer and pry it off. You'll be okay unless you pry it off in a weird spot I'd imagine. Just do it where it's unprimed at. A flathead screwdriver can also be used.
That is what is called a Back-Pressure airtank found in blasters such as the Hornet, Big Salvo, and a handful of more recent blasters whose names escapes me. I believe you can find a similar set up in certain kind of exhaust valves.
This link is the quickest example of a homemade "back pressure" firing system.
As for a more ergonomic ball valve, I'm sure there's a few creative ways you can work around them if you don't mind having fugly as a feature potentially. Here's something that might get the ole think muscles a whirlin'. I've always wanted to do something similar considering how cheap it would be to make something similar.