In most cases it’s better to max out HP and one defensive EV spread, but using your mixed wall example, it’s much better to use the second EV spread. I ran damage calcs and the defense boost far outweighs the extra leftover recovery.
252 SpA Choice Specs Charizard Flamethrower vs. 148 HP / 180+ SpD Audino: 166-196 (43.2 - 51%)
252 SpA Choice Specs Charizard Flamethrower vs. 252 HP / 128+ SpD Audino: 175-207 (42.6 - 50.4%)
Percentage difference per hit: 0.6%
With the first spread you get 24 hp from leftovers per turn, which is 6.25%. With the second you get 25 hp per turn (rounded down from 25.625 so what you said comes into play), which is 6.1%
Percentage difference per turn: 0.15%
Same thing for regenerator, it’s 33.33% with the optimal spread and 33.17% with the normal one, not a big difference.
So go with 252 HP instead.
Edit: I made a generalization that Audino’s always going to be eating up strong item boosted attacks, the other spread could be better in stall and double switch situations but what I said still generally applies