1- 100,000 Tier Points ($1,000,000 Coin-In at Video Poker or $500,000 Coin-In at Slots --- OR a LOT of high average table play!) [Platinum is 4,000 Tier and Diamond is 11,000 Tier]
After you hit 100,000 Tier they approve the level upgrade, based on you not having any outstanding credit issues etc. etc.

2- Hard to answer, buy itself, not really. Approximately 10x the play of Diamond, and NOT 10x the benefits. You do get a yearly $500 dinner (called you "annual trip") and some other perks (like the 7* lounges -- which are rather limited). You also have a 48hour comp-room reservation guarantee. VIP CHECK-IN (instead of Diamond check-in) where available and when open (in AC, VIP is ONLY open on Saturday and Sunday at properties like Harrah's). You "should" get the best available rooms (and often a suite), but once again, this is often dependent on your theoretical (with regard to the suites). With regard to regular rooms, getting the BEST regular room is basically a given for a 7*. First in line access to restaurants and clubs (though, this is seldom much better than the Diamond benefits). Reservation guarantee for all restaurants, but this can sometimes be a bit YMMV (and once again, isn't often that much better than Diamond).

Best benefits are still based on you having a stellar theoretical (ADT).

One of the interesting benefits, is that you can NAME a friend/relative as a 7* companion, and their card will also be upgraded to 7* -- but, it will (unfortunately) say COMPANION in big letters on their card.

3- Not sure what you are asking, but those who play more will inevitably earn many more RCs (Reward Credits)