Does % ele damage on items work off the spell type without a rune or after the rune?

Disintegrate (convergence) gets boosted by fire skills +damage% or arcane skills +damage%?

The tooltip when you mouseover a skill will tell you what type of damage.
It's been a while since I changed my gameplay settings but you should double check if the advanced tooltip option is selected in yours.

Convergence turns Disintegrate into fire damage so +fire skill damage will boost it even further.

See that's what I thought, but this guide for example says to gather arcane damage % while using fire damage arcane orbs.
http://www.diablofans.com/builds/67514-2-4-firebird-t10-rift-speed-farm

Skip that build. On the top right there a +3 (low rating).

Use this one instead: http://www.diablofans.com/builds/70020-2-4-wizard-firebird-beginner-60-easy-to-play-noob