For access without removing ypipe. etc, follow the how-to under duratec maintenance-->duratec faq...
I would have checked voltage at the battery before pulling the alt, but since you already have it.....
I would imaging that thw splash shield is the beginning of your problem. I suspect that the speedo/tach wierdness is from low voltage on the battery, with a no charge condition from the alt.. equating to low voltage at all times, even if you DID get the car started. The car does some freaky stuff at low voltage. (wipers are alive.. speedo/tach jumping or dying, etc)
the airbag light flashing could be the code that has a double meaning. one of which is "low battery", ironically enough, and would fit right in with this situation.
Since your alternator is out already, replace it with an Autozone "lifetimer" (that is if you don't want to use the old alt., which I wouldn't even if it tests fine...), verify the megafuse resistance (should be nill to none), then verify chargin voltage while running at the battery to be 14.2-14.7-ish.
This should eliminate the wierdness, the charge light, the no start, etc.
You stated that you COULD get the car to start by holding the gas down. on a fuel injected car this is not needed, but that is a way to test the operation of the IAC. I would clean the IAC as a precautionary measure, as it may come in to play sooner rather than later. (cleaning info is under the UIM/LIM rebuild aid under duratec maintenance's "duratec faq"
Ray
