Take all the swings you like, record your findings and report what you find.
Well, I did what you suggested. Vanillla settings across the board. Creative mode on to get the clubs and club mods. God mode. No perks used, no armor and I chose zombie Yo as my victim, testing with four wooden club iterations. 20 zombiess (one spawn at a time) for each upgrade of the wooden club. Fresh game, started at player level 1 and by the time I finished I was at level 9 with 109 zombie kills since there were a few interlopers during my experiments, lol. Each hit was a power attack. I let the zombie return to its feet before hitting it in the head again to allow for stamina regen. Yes, some were glancing blows, but not often. So I recorded the zombie's initial health which ranged from 120 to 171, and the number of swings I used to kill it. Entered my results into a spreadheet.
Lvl 1 club, 27.6 power attack rating : 75% needed 4 hits, 25% needed 5 hits (beginning at 150 initial health), with one decapitation on the last hit (136 initial health)
Lvl 3 club, 30.5 power attack rating : 25% needed 3 hits, 65% needed 4 hits (beginning at 140 initial health). One needed 5 hits (glancing blows I suspect), and one decapitation on the first hit (155 initial health)
Lvl 5 club, 33.1 power attack rating : 90% needed 3 hits, 10% needed 4 hits (beginning at 146 initial health).
Lvl 6 club* 40.0 power attack rating : 65% needed 2 hits, 35% needed 3 hits (beginning at 146 initial health). One loot bag drop. *club mods were weighted head, chain, ergonomic grip and the burning shaft. I didn't think to write down how many caught fire, but that wasn't what killed them.
My take away from all this? After spending an afternoon to eliminate 80 test zeds and analyze the results, my gut feeling that hitting them while stepping away was inflicting more damage is not reflected in the data. Also I never realized that zombies spawn in with an RNG health value range and the stronger ones generally did need another hit on the head. Since I was in god mode, the few hits I got from the zombies didn't affect my ability to swing back. My test was a bit tedious but easy enough to gather the data. I could have also invested in perks, used a maxed out steel club and continued the assesment, getting more decapitations per number of zombies eliminated and generally be able to one-hit a variety of zombies and send them flying. I suspect RNG also affects how much damage is inflicted. But I'd seen enough for now.
That said, this doesn't answer
@enkephalin07's other question about getting hurt whilst in motion. I don't think I'm going to try and design that experiment any time soon.