Transactions on Engine are not distributed across a network... Engine is not tamperproof.

Well, I see it differently. The transactions on Engine are directly the custom_jsons, where certain rules (public rules) are applied. And these jsons are distributed and cryptographically signed on Hive (or Steem).

Without a doubt, Engine is not decentralized.

Yes, I agree with you. This is the main issue of Engine. One could think that it is decentralized because it is just picking some transactions from the Hive blockchain (which is decentralized). This is true. However, it is not decentralized in terms of the number of nodes running Engine. And the reason for that is because only one user has the incentive to run a node: The creators of Engine (fees for creating smart contracts, tokens, etc). Apart of that there are no incentives to run a node. I'm mean, this is their business model, which is ok, but it was not designed to be decentralized.