^ if we assume that acetaldehyde is primarily responsible for the hangover, and that ethanol to acetaldehyde metabolism follows zero-order kinetics (the speed is constant, regardless of ethanol concentration*) for the most part, then it would mean that in order for there to be significant difference in ethanol -> acetaldehyde amount, ethanol has to exit the body proportionally a lot more if one drinks slower. I don't know if that holds water. On the one hand, what you say sounds logical. On the other hand, if you drink/absorb ethanol faster, then your BAC is higher, and urine (and other pathways, such as breath and sweat) should proprtionally contain more ethanol (at the same time, liver oxidation pathway is still at the same speed as slower drinking/absorption), which would decrease the overall acetaldehyde per unit of ethanol formation.
This is what confuses me so much, and I've even considered that perhaps the culprit is ethanol as well after all, not necessarily only acetaldehyde. Ethanol does interfere with many processes too, so I think it's not that far-fetched that higher BAC (albeit lower duration) could also result in a more severe hangover. What also led me to believe this is the fact that if I drink the same amount of ethanol on 2 occasions, on one I drink significantly faster and thus get a hangover, the hangover lasts well into the next day, up to 24 hours since my first drink; whereas if I drink slower (same amount), I'm symptom-free as shortly as 12 h since first drink. Now, logically the metabolites and everything should exit my body faster if I drink faster, right? Then why do the symptoms last longer? There has to be some downstream changes in the body which persist long after everything has left my system, so it is also possible that ethanol itself is responsible for this, which could explain why higher (but shorter) BAC is worse than lower, but longer BAC.
*I don't know at what concentration this begins to be true. However, considering the fact that ethanol is consumed in astronomically higher quantities (up to or over 1000 times) than all other drugs, I think it is not unreasonable to assume that zero-order kinetics start pretty much as soon as the first 10-20 ml of ethanol (~200-400 ml 5% beer) are consumed.