ENT didn't need to explain the forehead inconsistency. In truth it was DS9 that turned it into an issue by calling attention to it in "Trials and Tribble-ations." When the Klingons' look changed starting with TMP, Roddenberry wanted people to just assume they'd always looked like that, even during the TOS years. The ENT writers decided that there could be an interesting story in there and they decided to run with it. Personally, I rather enjoyed the Klingon Forehead Arc, I thought it was a pretty clever way to explain it.
Speaking of "Trials and Tribble-ations," I think it would have been better if they'd just put Michael Dorn in TOS-style Klingon makeup while the Defiant
was in the 23rd century, and had none of the crew notice any difference.