Yes, for multi attribute/element attacks, +character affects all while +element only affects the respective element. The textbook example the wiki uses is base Amon's fire/dark nuke. +fire character stuff would apply to both the fire and dark hits. +fire element stuff would just affect the fire hit. At least Solomon's multi element nuke is 6 hits, so +ability damage helps.
Probably need debuffs, but 2 million is....feasible. The first (single element) nuke soft caps at 500k. The other nuke caps at 100k per hit * 6 = 600k. Casting both twice soft caps at 2.2 million.