So I've hit the damage roadblock on an Archer

[Set]

While you may not be able to multi-class over into Scout or something, you might be able to work out a Feat option with your DM that gives limited Skirmish die-like abilities.

One that would be particularly suitable would be an 'Aimed Shot' Feat that allows you to sacrifice iterative attacks with archery to add dice to the single shot taken as a Full-Round Action. A 12th level Ranger would normally have 3 arrows per round at +12/+7/+2, but with this Feat he could fire a single arrow at +12 that does two extra dice. He'd only get all of his plusses to damage twice, and the Feats prereqs would be something like BAB+6 and Precise Shot. [An identical Feat for melee fighters would also make sense. Focused Strike would have BAB+6 and Power Attack as prereqs, and allow a Fighter to do the same thing, making single big sword hits instead of rolling a bunch of iterative attacks.]

 

[Plane Sailing]

The elemental burst enchantments do scale with crit multiplier so at a x4 multiplier they will give you 3d10 on a crit (eventually hitting 4d10 at x5 multiple) and do include the d6 elemental damage as well. You might be slightly better off on average with two elemental enchantments,

Mathematically two different elemental enchantments are always going to come a long way ahead of elemental burst. In order to be equivalent, the elemental burst would have to do 10d6 damage on a crit (really!)

 

[Will]

The crit thing is something I noticed when calculating damage capabilities of melee weapons: crits represent a very small amount of damage.

Specifically, a falchion is worse than a greatsword despite better crits, because the greatsword's effective +2 damage is a lot more than the fractional crit would be. At least until your Str score is in the 50s.

I call this 'reason #50 for why game designers need to be good at math.'