1)Using stabs... hmm. It's hard to say. First off, phantoms run best with their stock parts at 850psi. I, however, use a WGP regulator and run mine at 750psi. Why? I happen to have a high-flow powertube, so now my phantom works slightly more on the "lower pressure, longer valve dwell" spectrum rather than the "higher pressure, shorter valve dwell" spectrum, dig? Realy, with the stock parts on there... I may think you may just want to set the output at around 800psi or so and basically use it as an expansion chamber. Even though it doesn't doo a lot of actual regulation, it will convert any liquid to co2 and thus yield better consistency. To tell you the truth though, phantoms work great right off of co2. A regulator will only help when properly configured to the gun. Double regging may be a bit too much if you're on co2, because in colder weather if the tank pressure drops below your regulated output then your reg isn't really regulating..
I'd suggest slapping one of them on there- doesn't really matter which one, and trying your best to stay above 750 psi. Even though you CAN run your phantom at around 550, you are going against the design of the flow rates of the gun... just stick around 800 with the stock parts, alright!?
2) How many shots you get depends upon a lot of things. If you've got a really nice unported barrel right around 12.5" with a low drag coefficient and a super paint match, you can get killer efficiency. However, it really depends upon a lot of things. I personally never really counted, it will also depend upon the fill, temperature of the tank.. a lot of things...
3)I think you're talking about the centerfeed. Most hoppers just slip over it. I personally use a right feed and a VL-100 with a VLbow.
4)Important things to know that are not in the manual:
-Don't overtighten your nylon setscrew (the little setscrew thing in the bolt), this will strip the threads! I learned this the hard way
other than that I can't really think of any...
5)The only stupid question is the one that was never asked *wipes away tear*