On hard power switching the burner. I checked the boiler installation instructions and it takes a straight mains feed, so when it's turned off, it's turned off. I have read elsewhere you 'can' wire your heating water pump separately and have it run for several minutes after killing the boiler, but I don't think it's necessary. There should be some convective flow anyway. A previous house I lived in had a coal fired back boiler and the heating pump was optional, without it the radiators still heated, just much slower.
It's a kerosene burner, so unburnt fuel from a failed ignition shouldn't be an explosion risk it will burn off the next time the boiler gets up to temp.
The gas combi boiler I had in my last place had an anti short cycle to allow a failed ignition to disipate out the flu and it would continue to run this for a minute after before turned off. (Unless you pulled the mains feed).
So I think I'm good on those fronts.
For Wifi failure I can handle that in software. For software failure I could do something in electronics, but the relay being 5V high to close is a start, meaning if power fails to the ESP the heating goes off. If software failed, locked up, however with the 5V stuck high that might be more tricky and run the heating constantly. I could instead send a pulsed signal to electronics that would only close the relay if there is a pulse, a constant 5V would open the relay. As to designing that, not sure, but current across the secondary of a small coil/xformer might allow me to charge a capacitor to hold the relay coil closed, if that pulsed 5V become a solid DC 5V the xformer would not transmit current and the relay would open.
That only leaves the anti-freeze mechanism and as the current system does not provide such a feature and in this part of the world the heating loop contains anti-freeze I should be fine. I can rely on software, noting the garage temperature and run the heating if it's below freezing. This will be the coldest part of the house and "unheated" although it does contain the boiler, so running the boiler should, maybe, help keep it above freezing. It does contain the laundry equipment which is a consideration for freezing. An oil filled electric radiator might be a better option for the garage.