Hmmmm.... Here's the thing.... Unless almost all the exchanges colluded in unison, they would be screwing themselves. Any price fixing in either direction would immediately invite arbitrage at their own expense.
100% agreed, another reason for the Index
I disagree. If several exchanges colluded "strategically" it could happen. [Once again, this is just hypothetical...]
Although some will be able to take advantage with arbitrage, the transaction times of bitcoin end up helping the exchanges. Most people don't have enough funds spread through multiple exchanges. Think of this as a sort of dispersed "pump" or something.
Admittedly, we see this sort of thing with other exchanges dealing in alts (where there are fairly large spreads between exchanges). Although, relative illiquidity is the culprit in those scenarios.