It's possible he was half-turned in DB, though I don't think it's necessarily the case.
I would note though, Harry described his magic as dark but definitely human as of DB (unlike Grevane / Corpsetaker / vampire sorcerers who used the 'cold and greasy' power source), so he can't have been any more than half-turned. And as far as I recall the battle scene in the Deeps in WN, Harry got enough of a sense of the opponent's work there to be sure it was Cowl again - were he a full vampire at that point, I don't think his 'signature' would have been still so identifiable. I think the Darkhallow backlash wounded him, but not severely enough that he'd need to embrace vampirism to recover. Indeed, the fact that he was still moving as though pained in WN speaks against a full transformation to the Red Court, since their much greater healing abilities would have had him fully recovered in a matter of days. It probably even weighs against him being half-turned, since he had plenty of time to get his hands on a non-fatal increment of blood to restore even a major injury without turning (as Harry donated to Susan to fix her broken spine after going up against the Ick).
More likely, I think Simon would just be at or above Ebenezar's level of proficiency at enchanting passive defenses onto his clothing. I think he just tanked through the backlash with advanced armour enchantments Harry had no idea were possible at the time (it would also explain the car flip catching him by surprise and still not hurting him).