Compared to what? Permanently venting air is a much bigger waste of energy.
Most of the heat is in the walls, not the air anyway.
Since forced-air heating is uncommon in Europe the impact is not that high. The radiators or under floor heating stays warm and will re-heat the air very quickly.
No, not necessarily. If it lowers the humidity, then it can take less energy to heat the house after venting. So it can actually save energy.
Minimal. Most houses have thick walls and there is a lot of accumulated energy - the air reheats quickly after closing the windows.
5 minutes of cool air entering the house isn't a huge amount, but it does bring in fresh air.
Yes, to a certain degree it undermines the point of having good insulation (as in, really good - when the thermal mass of the air starts to matter). Modern "nearly zero-heating" houses have long heat exchangers in the vents to avoid the waste.