What does the acid do in this reaction?
My guess is that the acid is just there to protonate the carboxylate salt and allow decarboxylation. Hydrolysis of the ester with $\ce{KOH}$ generates the potassium carboxylate salt:

Since the mechanism for thermal decarboxylation involves a cyclic transition state including the carboxyl $\ce{-OH}$ group, I expect that the carboxylate salt must be protonated before decarboxylation can take place, hence the addition of aqueous acid.