1. The classification boards for the country e.g BBFC in UK, ESRB in US
2. Don't really know the answer to that but they must have known it was heading for a high rating.
3. The BBFC cited Manhunt 2 is distinguishable from recent high-end video games by its unremitting bleakness and callousness of tone in an overall game context which constantly encourages visceral killing with exceptionally little alleviation or distancing, and There is sustained and cumulative casual sadism in the way in which these killings are committed, and encouraged, in the game," it added, and highlighted "the game's unrelenting focus on stalking and brutal slaying and the sheer lack of alternative pleasures on offer to the gamer.
4. Yes in the UK in 1997 a game called Carmageddon was banned also GTA had to be recalled and changed after the famous 'hot coffee' incident.
5. In the UK the BBFC has said no amount of chopping and changing will see it released in the UK.
Hope some of the information has helped. It is my opinion that the BBFC has got their verdict completely right.