Yes, the length of the tournament may be a problem - but I don't think 15 or even 16 or 17 days is too long (Euro 2012 will be 24 days long, for example).
The 6 teams that are eliminated in group play and in the crossover quarter-finals will play only one more match than they do at present. The 4 semi-finalists will play the same number of games as they do just now.
About fixing games, this is of course possible. But if CONMEBOL was really worried about this, they would have simultaneous kick-off times in the last round of fixtures in the 4-team groups.
Technically, any 5-team qualification group for the World Cup, for example, is open to possibilities of match fixing. And there have been many 5-team groups.