That doesn't exactly help you very much.

The idea of a transparent blockchain is to deter such conflicts in the first place since any party making a claim to have altered the blockchain state will have that claim arbitrated on by the entire userbase, not just by me.

Therefore, Poloniex can make any claim they like as to what state change they've made and that claim is instantly testable, not just by keyholders but by non keyholders also. Furthermore, that applies not only to the positive case but the negative (absence of state change) as well - no 'assumptions' needed.

Encrypted blockchains are designed to keep everyone else out of the loop except the sender and receiver. Thats what makes them ok as an encrypted messaging system but valueless as a monetary asset in their own right (an unbacked one at least) since bearer tokens don't distinguish ownership from possession. The encryption is therefore redundant and only serves to obscure (and potentially corrupt) their authenticity as you've already illustrated by demonstrating that the co-operation of 3rd parties (keyholders) is required to fully verify that a claimed state change did in fact occur.