Reduce heavy whipping cream for a sauce a day ahead of use

Heavy cream can be frozen but only if intended to be used in its liquid form (soups, sauces, etc.) It will not whip properly once frozen.

I agree with Pointy that there's really no point in freezing it because it's usually dated about 2 months out from the time I'm purchasing it. Additionally, if kept cold and not left out on the counter unnecessarily (as with most dairy products) it will keep well beyond the date on the carton.

I'm not a milk drinker but use it baking/cooking. If I have a recipe that requires milk I use a 50:50 ratio of heavy cream and water and haven't had any issues yet. If I was using milk it would be whole milk so it's kind of like making your own version.

Unless there was sugar in your cream, there's nothing particularly sweet about this combination. Did you salt the pasta water to roughly seawater saltiness? If not, did you adjust salt before serving? If your pasta water was under-salted, that would explain the sweetness, since the only significant source of salt otherwise was the parmesan.

Heavy cream is equivalent to 36-40% whipping cream.

In the event that your cream was pre-sweetened, there's not much you could do except add salt and hope the sugar doesn't overwhelm. It's a fairly common technique to add sugar to a salty dish to make it taste richer, or salt to a sweet dish for a similar purpose. But it wouldn't work very well if the cream was very sweet. Since I've never seen sweetened whipped cream except in aerosol dispensers, I can't say for sure.