Well, the heater power switch for any home comes with N already what. Standard

If you tell contractor where you want to out switch for water heater during renovation, automatically that switch location will be meant for 20A DPST with N available. The smart switch for 20A usually require the N.

I'm not sure if I really understood what your worry about.

The value of capacitor provides reactance value that will determine how much current flows to the switch when the smart switch in OFF condition. Reactance is like an AC counterpart of DC resistance which is also measured in ohm.

Given the AC frequency of 50Hz and say 0.1µF of capacitor value, the reactance is about 32kΩ. Therefore the current that flows in OFF state is only about 7.8mA which is used to power the smart switch in standby mode.

Ceiling fan is low power device. Mostly around 50~70W only for old model. New one with DC motor even less than 40W.

Some hole deep and some too shallow.

Yes, when I put additional sockets it became too cramped. I had one that I dug a bit deeper to recess mounting bracket deeper inside just to make room

It is not about the shape but the square pin is rated at 13A because it is the amperage that it is designed to handle. So, if you use higher load, the plug top will get hot and possible get that burnt smell and perhaps damage the socket and plug over time.

Note that the round 15A plug has bigger pin too I believe.

The fuse is for safety purpose, not to rate the plug. It is the standard that require the fuse to be there. Last time the UK also use round pin but later changed. Actually the square one has more contact space and gives lower contact resistance according to what I read.

Obviously it is just what you thought it was

For lighting points, normally 6 lighting points (if not mistaken) will share 1 MCB (mini circuit breaker) of 6A. One single N wire is looping to all these points and return to the DB. The 20A switch will have its own N wire going back to the DB. This is because it is meant to carry the 20A load back to DB from L wire.

Even though it is a single phase wiring where the N is common throughout the house, the N wire to devices are separated throughout the house because of the current load wires have to bear. So, when you loop the N from the switch to the 20A switch, it will work for single phase usually, but now the load from smart switch will now share the load of the 20A switch.

If I understood the smart switch with N requirement right, the N will just carry very small current just to power the smart switch, not the load from light points because they have their own N at the ceiling. Therefore, I think this will not be a problem. No, it will not cause any damage to house wiring anyway. The 20A MCB still protect the wiring of the 20A switch.