I've not seen the Klingon Forehead Arc, but the show hogtied itself putting a bumpy Klingon in "Broken Bow" in the first place. If they'd started with smoothies, there would have been no need to explain STTMP Klingons,
The Klingon arc wasn't presented because of a "need" to explain anything. They were presented as another in a series of season 4 stories which dealt with the origins of many things Trek fans take for granted about the Trekverse. Ent was a prequel after all. I thought most aspects of the arc were great and the presentation of the forehead pretty well thought out and plausible.
But, I forget. You are likely among the fans who identify prequel stuff as "fanwank" and therefore, unwanted, right?