1. Normal fermentation in a closed vessel would quickly put it up to 50+ psi, not terribly safe. Theres a lot of CO2 given off during active fermentation, at 7psi I can actually hear the faint hiss of CO2 slowly leaking off.
2. No the natural production of CO2 is giving me all the pressure I need, the adjustable pressure relief valve (spunding valve) bleeds off whatever extra pressure while maintaining what I have it set at. The 7psi is supposedly the kind of pressure yeast feels at the bottom of a large conical commercial tank.
3. Dale Penrose said they are tested to 160psi, and I've heard of a few people carbonating in the vessels. Some have even served from them, although its more common to counter-pressure transfer to kegs. You can actually use the CO2 produced by the fermentation to prepurge your keg, just leave it attached via a ball lock QD hose for a day or two.
Check out the wiki on pressurized fermentation, or the very long thread on that other homebrew talk site. I'm a poor person to talk about it other than I have maybe the ultimate vessel for this method. It can be done with cornies or sankes as well.