1) The PTS PMAGs are good quality, but no real advantage compared to others. You could pay £30 for two PMAGs but you could get a pack of eight MAG (the brand) magazines for £40.
2) Answered
3) Gas, you can leave it. CO2 no way (learnt the hard way with a couple £40 Palco USPs)
4) Depends on the manufacturer! WE are notoriously unreliable and known as one of their worst pistols. TM on the other hand are excellent as usual, plastic slide means gas efficency is far superior (and of course performance is excellent too because it's a Tokyo Marui) I have heard a lot of times other 1911s can't even shoot off a whole magazine. IMO TM 1911s / Hi-Capas = only ones you should buy.
5) Not good practice to be leaving a gun loaded! Real or not.