The miner could just ignore tx A since it doesn't add value to this tx.
Can he accept this transaction as good even if he doesn't have tx A?
The fee is not specified directly by the transaction. It is calculated by looking at the difference between input value (which is also not specified) and output value.
Ignoring the other problems with doing this, you could accept the transaction into your own blocks without knowing the exact fee, but you could not correctly verify other blocks that include the transaction. For all you know, the transaction you've forgotten permits a fee of 1000 BTC.
I would balance the risk of wasting hash power by accepting invalid blocks with the gain in hashing power for not bothering about old transactions.
You don't gain any hashing power. Only disk space, which is cheap.