I can't answer all your questions but I'm pretty sure that storing potions don't inflate the bills as they sell for nothing so basically have no value. Angelfish have minimum value so again, don't inflate the bills by much. Cooked ambrosia has quite a value so storing these could potentially inflate your bills quite a bit.
I store angelfish and potions on the roof or in the basements. (Now that basements are here I won't use the roof ever again for storing items! lol) I've around 130 potions and about 120 odd fish. I keep the death flowers in the personal inventory of my founder. (Have 99+ so I don't actually know how many I have). The main reason I do this is so that the angelfish aren't used up accidentally by cooking fish tacos (which my sims live on) and also so that I don't end up somehow accidentally cooking ambrosia, leaving it on the bench and end up having a ghost eat it!!
I don't spend my founder or heir's points on potions until they actually need a plate of ambrosia, that way it slightly reduces the amount of potions in the basement. Also, just because I like to try to make it so every heir 'pays' for their own potion. Ambrosia seems to be one of the quickest food items to cook so my sims just cook it one plate at a time as they need it.
Last thought, ambrosia doesn't go off anyway, regardless of who cooks it.
Not sure if any of that's helpful or not but it's just how I do it!