1. this horserpower comes from voluntary contribution of members anywhere around Globe. and most of computers - usual home PC's. which can be or canot be used for this purposes from time to time.
2. its fair. BTC only lack more functional software to use this horsepower for other[than hash-crunching]. true for nearly ALL highly-paralell computation with DP-precision-hungry-requirements.
3. sure, you are. and welcom.

For doing the type of calculations it does, I think you can safely assume there is no network out there as powerful as bitcoin. But the type of hardware mostly used for hashing blocks might not be that effective when used for other types of calculations, so depending on what you need to do, I'm not sure the bitcoin network is the strongest one available.

Regarding variation, estimates of the total computing power are done using the number of blocks produced in a time window and the difficulty factor. If the time window is too short, like a few hours, you'll see wild variations. You should get estimates which use larger time windows, like one or two weeks.