if SINGLE privkey is compromised from linear chain then ALL privkeys can be computed in same chain.
that's not right. you'd need more information, like the chain code if it's even possible, than just a single privkey from the chain.
you need the master seed and one privkey if I understand correctly. Not sure what kind of other information would be sufficient, but a single private key isn't.
shown here by Vitalik:
https://bitcoinmagazine.com/8396/deterministic-wallets-advantages-flaw/If you have the master seed can't you already generate every private key?
The main question is if you have just the private key for one or more addresses in a deterministic wallet, are the other addresses at risk or are they secure.
My understanding was they are still secure.