It says "when played". So he should count himself when on the board imo.
Wrong wording I think. Count Caldwell won't count himself in the later turns (although he has been played before).
(My English is not too good, I hope, you understand what I mean.)
"Units with the Dominance ability can no longer count themselves as the highest units when played."
Think of it this way, when you're playing a card, it doesn't count itself, thus the board state is checked for highest unit. If you succeed, then you trigger dominance ability.
Once your unit has resolved, it now is on the board, and counts for future checks.
Seems consistent to me at least... To me the deathwish/banish wording could be clarified a bit better, but overall I'm happy with wording.
Caldwell has not the dominance tag. The reported situation is weird imo
I think "when played" in the post meant deploy or order. The card never counts itself. If you have 2 Imlerith, one the biggest unit but the 2nd biggest is the opponent's, then only one will do 2 damage (the smaller one, counting the other as the biggest unit). I know, weird scenario but it actually happened to my opponent 2 days ago.