IPhone – Why does the iMac charge the iPhone faster than the wall charger

Your assumption is correct. Your iPhone has an intelligent charge controller on-board and this controller will only draw as much as it needs.

Sure, the iPad power supply can supply twice as much power as the iPhone needs, but it should not be dangerous by any means.

I don't know if there is an "official" word on this, but I would be genuinely surprised that it voids your warranty. Apple engineers are very smart and know that users will interchange powersupplies if they recognize that the connectors are physically compatible (as is the case here.)

Long story, short: you should be fine.

No

I mix and match my chargers often, the only disadvantage is that the iPhone charger charges slower according to a lot of sources, but I have yet to experience it. I still have an iPhone 3G charger I use as a spare at work to charge my iPad on occasion.

Interestingly I have never noticed my chargers getting hot either, but will admit I have never actively looked.

Looking at the Charging Tips here the reason for the slower charge is that the iPhone charger is 5W compared to 10W for the iPad, based on wattage alone, I can't see how you would damage the iPad, I would suspect using a higher wattage might however.