I'm aware of the safety features, the pic I posted of the steam split fitting came off a boiler with safety features, I was at a combi that is way more complicated couple of weeks back and that did exactly the same with all it's safety features, woman thought it was going to explode, things happen.This water heater does have safety sensors, inc one for if the fumes/emissions going outside norms.
Plan is defined to inhibit the heater from being able to start when not required (I.e. loop an interruption switch by the sink to disconnect the ignition circuit supply when not in use. I will also have the main volume water pump on a similar circuit).
I can see that the flue possibility is a non-starter given what's been said about the exhaust temps (this is not unexpected, but worth asking the question anyway
If you restrict the flue or don't get rid of the poc you run the risk of the flue spillage nocking the boiler off.
I'm sure a setup where it's reasonably safe is possible I'm just pointing out the risks of relying on safety features, it's a bit like relying on a co alarm, it should work but you wouldn't use it as backup for something used in a dangerous way.