The Mercedes were quick last year over one lap, not to the extent that they are this year, but as you say, it's over a number of laps now where they have the advantage as they have learnt how not to chew through tyres. If they had sorted this last year then Vettel would have had a harder fight of it.
I think it's more to do with how they have their ERS set up, they can use it all lap every lap, whereas the Ferrai's and Renaults are only able to do it every other lap, and thanks to the rules they can't change their systems until 2015. Pirelli's harder tyres have helped a bunch too, but I think if it were just the tyres someone else would've caught up by now.
there are minimum weights; overall, and front and rear, as i said...and??? i don't see where else weight distribution of the car (not just the engine on its own) is "strictly regulated and controlled", maybe if it were there wouldn't be multiple seconds a lap difference between the cars at the front. I don't see what difference it makes, Mercedes clearly have huge advantages in multiple areas of the car, if not all of them. What are you trying to imply, that they don't have a huge advantage?!