Doubt you would have the same view if it set fire to something, highly possible if something failed.
Input is AC so no positive or negative...
Alarm cable would make sense for everything on ELV side, unless you have data transmitted then you will likely need a TP cable.
PSU's don't "emit" 3A they "supply". Each core of good quality alarm cable as a max current of 1A @ <50V max.
Therefore, I would be using all the wires of an 8 core, if you are crazy and plan to pull a 3A load from this cheap supply for whatever reason.
I'd think an extra 50 quid or so would have got something safe, that comes with a understandable manual?
Although the text on the terminals seems fairly straight forward.
I reckon the cheapest option would have been a mechanical solution on the cupboard.