Regarding sharing a full game - I don't believe this is exactly possible. PSN games, yes, but not full games.
I'm basing this off the fact that I downloaded Warhawk on release, but 1 year later changed accounts and PS3s. Downloaded it again using original account then tried to play on new account - it was a nightmare.
It did work, but it's loicked to 1 account at a time. So if you shared the game, you couldn't both play it at the same time for instance.
That was only 1 game - but I would assume full retail games did something similar.
PSP games are extremely pricey yoo. £28 when you can pick it up in shops for £10 (new).
Regardless of price, I wouldn't even want to imagine how long it'd take to download on my internet connection. If I wanted it on release day they'd have to let me start the download about 4 days prior!! lol.
I could drive 25 mile round trip to Newcastle and buy it there 10x over by the time it would take to download