Can `sign_with_ecdsa` be forged?

I believe this question is already answered in your linked thread, i.e. one corrupted node can’t forge tECDSA signatures or do unauthorized requests.

Quoting the developer docs:

The Internet Computer implements a novel threshold ECDSA protocol as part of its chain-key signatures suite. In this protocol, the private ECDSA key exists only as secret shares held by nodes. Secret shares are shards of the ECDSA private key. Signatures are computed using those secret shares without the private key ever being reconstructed. Each replica of such subnet holds a key share that provides no information on its own. More than one third of the nodes are required to generate a threshold signature using their respective key shares.