Can I make a hollandaise sauce from mix the day before?

I don't think it will work... at least reliably work.

Point of reference: I once worked in a restaurant under a frugal French Chef (I know, a redundant term). The only thing that was always thrown out at the end of the night was the Hollandaise, no matter how much was left.

Hollandaise is a delicate chemistry of proteins, fats, carefully applied heat and whisking. Surviving the create with heat, cool down/store, reheat cycle is a long shot. ..and, as you suggest, there may be safety issues.

Of course the restaurant made Hollandaise from scratch. But I doubt package mix is much more robust. Note these ingredients on McCormick mix:

Those egg solids are going to be delicate, and maybe unpredictable. The lecithin may also bring surprises. The citric acid may also cause issues in storage--maybe breaking protein/fat bonds.

Or, I could be wrong. Maybe the high amount of starch will make the sauce robust. Example: you don't hear of many fruit pie fillings breaking overnight.

I'd spend the money and do a trial run to know for sure.

End Note: Consider Knorr brand Hollandaise mix. I never used McCormick, but I have used Knorr and liked it.