Modern engines don’t come up to temp properly until under load.
You need to drive the car for it to be efficient.
Most heaters have an electrical element in them no as we all expect instant warmth – not 15miles down the road like an old ford…..
Leaving it idling will damage the engine as the warm up is localised and can lead to all manner of damage and wear.

Bearing in mind that it takes about 30s to melt the ice with a heated screen, I don’t think the idling time is really an issue.
I have to wait longer in my non-heated screen car to de-mist.

I thought cold water was best as its got a lower surface tension than warm and doesnt freeze as quickly?

Cold water doesn’t freeze as quickly as warm water? Are you on drugs?

That’s a bit harsh!
Surely the question should be whether his rear tyres have more tread on than his fronts on his FWD Hatchback? 🙂