Can Accelerated Rot deal 9 damage if you meet elemental requirements?
Yes, major powers are very strong when you hit their thresholds.  I don't have a rules citation beyond what you list there, and I don't see anything explicitly in the FAQ about it, but the lack of "instead" and the plus sign both indicate that the power does 9 damage and removes a blight when played and you hit the threshold.

Compare to Pillar of Living Flame (3 fear, 5 damage, add blight if target land is jungle/wetland), which by the other interpretation would get worse at the threshold (+2 fear, +5 damage).