1. When did they merge? I know it was possible to use blockchain.info for logging in sometime earlier but not anymore
2. Was it possible to use Bitcoin address as an ID? If so, what was the principle behind it? Was it possible to use
just any address i.e. 1BitcoinEater..... as your log in info or was it only possible to use an address that was provided to you by the BC wallet?
There is no relationship between blockchain technology and that site, they use the distinguished name to promote their service, but it is a wallet that you register with your email and you are given a Wallet ID with a password and access to your wallet.
12 words wallet seed is another option to recover your wallet.
If you have an old account in that wallet (until 2013) then log in normally, and if it does not work, you will need to click on this link
https://login.blockchain.com/wallet/import-wallet