I am not aware of any study on prophylactically use, but I am cautiously hopefully that it work based on personal experience and from what I hear. For water sport activities, flushing with clean fresh water, dry it, and then spraying an acid solution seems to give some defense in keeping outer ear infections at bay. Alcohol could help drying it out, through because of the risk of triggering inflammation in the middle ear I personally avoid it.

I don't know why the studies shows them to be ineffective. Since they include acid drops in the studies it seems to be a well known home remedy, and given that it used prophylactically by the military in several countries there seems to be some reason for it. I could make a guess that doctors in the military/pediatric care are already keeping the ear of the patient dry so that alcohol don't provide any additional benefits, and they remove vax/debri in case those prevent the ear from drying.