HD wallets as described here are built based on a master seed and a master public key as follows:

PK.b = PK.a + sG
SK.b = SK.a + s

1. I just got a little confused about this. Is s the mnemonic seed that user knows? which generates the private keys.
2. Secondly a service provider only needs to know sG to calculate the user's next address. Am I right?
3. And lastly more importantly I am just wondering whether it is possible to calculate the master private key (seed) if an attacker or service provider gets one child's private key address?