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).
No they are not, those are just deposit slips (an unofficial unauthorative request for a transaction), the transaction actually happens behind the scenes on the "side chain" in a mongo db. Anyone can make a custom json with any number of tokens regardless of balance.
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This is the reason I added: "where certain rules (public rules) are applied".
Sure, you can create wherever custom_json you want with any token regardless of the balance, and this will be a valid transaction on Hive. However, certain rules apply on Engine, and they are public rules, then that json will be rejected there.
This transaction is not broadcasted because there is no reason to do that, it is already on Hive.
In any case, I agree with you that Engine can not be considered a blockchain because there is only one node running it, there are no incentives to run more nodes, then it is centralized.
There are no incentives to nodes on Hive either, at least RPC nodes (which is what Engine nodes are). There are incentives to run a witness node, but full nodes are much more expensive and difficult to maintain but there is zero incentive. Every full node on Hive is a blessing.
Yes, you are right, this is a problem. And in many cryptos, this is an issue as well.
The lower RPC nodes, the greater the risk, the less trust in the network because there are few providers to verify the data (unless you run a full node yourself).