Balancing is gonna always be an issue since people are never happy with what they have. They don't want to give up anything in order to improve other aspects either, they just want more and more.
For actual suggestions, I realize modifying durability might be out of the question at the moment? I'm not really sure how to address DaShniper's request since I'm still a little confused by it, besides making mobs have slightly less hp or upping damage for crafters.
As for eisbaer's request, it sounds like he just doesn't want to have to use diamonds to fight. So maybe, just make iron pickaxes do same damage as diamond. Crafters will be losing a real "end-game" weapon but it would make resource consumption easier to manage. This would make the only incentive to get a diamond pickaxe to be it's superior durability.
However, to increase an incentive for a diamond weapon without actually increasing the overall offensive power of crafters would be possibly... Adding a slight bonus to experience gained when killing with a diamond pickaxe versus an iron one? Just something minor like 10% or maybe even less? Or a higher chance of acquiring coins or an increased range of coins when killing mobs with diamond? The coins thing would follow along better with crafters anyways. Not sure if this is possible, but it's the best I could come up with for now.
Also, sorry for these walls of text.
TL;DR: Dunno what to do for DaShniper's request. For eisbaer's request, make iron pickaxe dmg = diamond pickaxe dmg. Add exp or coin bonus for killing with diamond pickaxe?