The game does in fact seem to have disappeared from the "for sale" area of the store, but (weirdly) the demo is still available; if you're interested in buying the game, you could always download the demo and see if it gives you the option to buy it in either the XMB menu or the actual in-game menu...
And no, there is no platinum for the PSN version of the game; as far as I can tell, the only version of the game that has a platinum trophy is the French blu-ray release.
Online (versus) is probably the fastest. If you kill another human player it gives you 3+k or something like that.
If you prefer offline, then you should complete missions in the campaign or single missions on simulator difficulty.
But don't worry too much about the ranking up trophies, you should get them while going for the other trophies.
Thank you! managed to get it finally! now just to work out this vibrations one and it will be platted! good luck on you future tophy endevours and I am sure I will speak again (see you abouts here all the time)!