How to receive payments online to single address per user in scalable way?

Suppose we are building online casino/market and want to receive payments.

To be scalable, safe and have good user experience, we:

• cannot generate new address for every single payment. Having single deposit address is a lot easier. Also users always can deposit to old address by mitake even if generated new address.
• cannot check listreceivedbyaddress because regulary checking hundreds of thausands of addresses is not scalable. Also large RPC output when single user make thousand of deposits.
• we should do not rely on external services online blockchain API.

We absolutely cannot use any RPC call that will list complete and growing list of transactions like all transactions for wallet or all transactions for address.

So, the most sensible way should be checking by listsinceblock to parse all transactions "per analysed block" and joining to user balances by destination address - incrase user amount in our online service and mark this payment as "completed" to not count it second time. Simplest way of marking this transaction as "already resolved" is to store its ID as "already used" in same SQL transaction that incrases user balance.

But I heard that it is possible to change transaction id in block chain, so is such solution safe?

How can I use RPC calls to receive payments block after block and which data can I use to mark payment as resolved to be safe from duplicated deposits on single payment?

TL;DR how "big players" like Cryptsy do receive payments?