If you need to select enough materials to select all except one for an Alchemy Lab level, and it is transmutating between the same rarity, then the output is guaranteed.
Eg: I am manufacturing Backpacks with the proper level.
5A you can absolutely use to create just Mana pots.
My only caveat to 5A is it is expensive and only makes 2 per day. However if you have a midnight root shortage I can see the use (and used it myself)
Depending on your stocks you might find 2B better. You will burn through little Health potions as it’s 10 a time but you’ll get more Mana pots on average for cheaper food doing this way