Will it be possible to produce view keys to reveal properties of just one particular transaction, or will view key's reveal all transactions relating to one Monero address?
I can envisage scenarios where there is a need to prove a payment took place, so maybe view keys for for single transactions would be useful.
Yes. It would vary depending on if you're the sender or receiver.
For the receiver, you could release the key derivation (ECDH); this would allow the "unmasking' of the stealth addresses given your (the reciever's) public spend key. It doesn't 'technically' confirm that the ECDH was derived from your viewkey, but in practice it's *unlikely* that you're revealing someone else's transaction that happens to be using the exact same spend keypair as you.
For the spender, you can sign a message (probably hash of the tx pub key) with the tx priv key. You can also provide the ECDH (and receiver's public spend key) if you need to prove where it went.
Edit2: in practice, I don't think the software stores r (tx private key), so you wouldn't be able to prove anything after the fact (unless you specifically kept r on hand) as the sender regarding where the funds went (beyond as your own change).
Edit:
Will it be possible to produce view keys to reveal properties of just one particular transaction, or will view key's reveal all transactions relating to one Monero address?
The view key that's part of the address reveals all payments to that address.
Every transaction sent uses a transaction specific key, however, and that key can theoretically be shared with a third party, who can then use it to decrypt the transaction.
I'm not sure whether that third party could know which outputs are change, though (and thus the exact amount). The addresses the outputs get sent to would be one time addresses, and thus not directly linkable to a standard address.
Given the transaction specific key, you also need the receiver's public view key to compute ECDH. It's not so much a "decrypt" as "trial 'encrypt' and compare", but it definitely allows linking to standard addresses given the right information.
