It doesn't explain why 250% of the coins are not held by Dev either

Premine = the dev creates a coin where a number of coins (from 0.0001% to 100% of the coins) goes straight to a certain address and is then used/distributed etc.

Instamine = fast mining (where many are mining). Example = PoW/PoS coins which the entire monetary based is instamined within a week. Now, just because something is mined fast doesn't mean the dev has 100% hashpower. Go to the Darkcoin thread and look at the miners and their WTS 10.000 DRK for 0.25 BTC.