1) Not really. There are enough headphones in the price range that are more efficient than the Mad Dog that will be closer to their best without an amp.
2) Not really, and many will find they can't get it to a satisfying volume level without an amp. Not that I think it's a great headphone, but you can at least drive an ATH-M50 to a satisfying level straight out of an iPhone.
3) LFF's Paradox, the now discontinued Denon D7000, Fostex TH900 (and probably the TH600). All cost at least $200 more. While the last three can achieve a decent volume without an amp, they also sound better out of a good amp. The Paradox, like the Mad Dog, will also require a decent amp.
5) I think the MD is just barely on the dark side of neutral and very smooth. There's no real consensus as many are using different amps and sources and have different preferences. Just read through the thread starting with comments from around Feb 7th (that's the date the new pads were released).
6) I'm pretty sure this was posted just a few pages back.
Is there any reason in particular you don't want an amp?
Agree with all jazzerdave mentioned.
I find the Mad Dogs super underwhelming without proper amplification.