Mage hand can pour out a vial of acid.
Pouring a vial of acid in this way will (generally) not damage an enemy.
It's exactly as the rules you quoted say. You can pour out the contents of a vial. Per the description of Acid, simply pouring isn't enough to do someone--who likely doesn't want acid poured on them--damage.
If you start pouring acid on one who doesn't want to be acidified, they can move, interpose an object, or otherwise evade the damage. In order to damage someone with acid you've got to splash it onto them, an activity that's defined as a ranged attack. And Mage Hand, per its description, can't effect damage on someone trying to not be damaged: it can't attack.
Could one pour the vial over an unsuspecting enemy, or one incapacitated or restrained, thereby causing them damage? I contend it follows the same rules: if you would need to make an attack roll, Mage Hand can't do it. Whether those situations require an attack roll--and therefore are attacks which Mage Hand cannot effect--is a ruling to be made by the GM.
It is impractical to make the pond Holy using the ritual.
Assuming that the pond is perfectly cylindrical, it has a volume of 7,363 cubic feet. Google claims that such a volume is equal to 440,633 pints. All of the flasks listed on PHB 152-3 contain 1 pint, so if we assume that the ritual makes 1 pint of holy water, it would take as many hours to make all of that water Holy according to the PHB ritual.
A cleric of any level can spend at most 16 hours a day doing the rituals, because they have to spend at least 8 hours a day resting in order to get their spell slots back. At that rate, it would take 75 years and about 11 million gp worth of silver to turn the entire pond into holy water.
Unless I've made a serious math mistake somewhere, this order-of-magnitude calculation shows that it doesn't make sense to turn a large body of water into holy water.
However, the cleric does have the Divine Intervention class feature (PHB 59):
The DM chooses the nature of the intervention; the effect of any cleric spell or cleric domain spell would be appropriate.
It seems to me that making a pond full of holy water would be within the purview of this feature, though it's ultimately up to you as the DM to decide.
There are no rules for potency.
I could not find any rules that dictate whether or not Holy Water retains its powers if diluted. Therefore, it's probably up to you as the DM to determine this.
What's the point?
Currently, in the rules, holy water doesn't have too much importance; it's a somewhat weak attack against fiends and undead, and it's a component of some spells.
I think that it's likely that your cleric player wants to accomplish something specific with the holy water. You should ask him what he wants to do with all that holy water, and then work with him on how he can accomplish that particular goal, instead of getting hung up on specifics like this. I find that focusing on intents and goals, rather than whether particular methods are impractical or not, tends to have better results at the table.
Best Answer
This spell is the only official mention of "Unholy Water"
A quick search on D&D beyond, the official D&D 5e web toolset, reveals that the Commune spell is the only official mention of the term "unholy water".
That means we have to fall back on natural language
Since the term has no official game meaning (because it is only mentioned once as a spell component and nowhere else), we have to use the natural language definition of the terms.
Unholy water is, obviously, water; it's in the name.
Dictionary.com defines unholy as:
By this definition, unholy water could just be water the isn't holy water. However, this would mean that something like tap water could be used as the material component of the spell. Since, Commune specifically allows you to talk to a deity or divine proxy, I would argue that this non-religious/non-divine definition isn't what is intended.
So, based on the second definition, unholy water is some way the opposite of holy water. Where holy water is water that is blessed and carries a positive religious connotation, unholy water is profane, wicked water with a negative religious connotation.
RAW however, that means unholy water doesn't do anything
As you have noted, there are no rules for creating "unholy water". There are no descriptions of any mechanical effect that it has, or any uses for it besides this one spell. RAW, it is simply used for the Commune spell and nothing else.
It has no other properties RAW, and any other mechanical effect given to unholy water would be a DM call.