If I sign a message and so lose the SHA256, and RIPEMD160 protections for that address, now only having ECDSA,
are all the other addresses so weakened as it is a deterministic wallet or otherwise?
no they are not.
the only scenario where this is a problem is below:
- attacker has your master public key aka extended public key.
- attacker has the private key for any one of your addresses (private key not public key).
In the above scenario the attacker can enumerate all the private keys in your electrum wallet. They can get all the money.
For other deterministic wallets it is different because they use hardening at some levels. Hardened chains can't be derived if the attacker has the above information. There are downsides to this as well such as the fact that you can generate addresses from just the xpub which is a useful feature of deterministic wallets.