Underfloor heating: Are the pipes tight enough?

What also confuses me are the pipes laid almost like a "snake" or, in other words, like a serpentine? As far as I know, you should always lay them in a snail shape, starting with double spacing, getting closer, and then going back outward "in between." In your case, you even have areas with different temperatures; the water cools down along its path. The snail shape compensates for this to some extent. Regarding the spacing, we have a laying distance of less than 10cm in a Kfw70 house (built after 2009) with KS plus 16cm insulation, and I operate the heating at about 30°C supply temperature at 0°C outside temperature. However, we also have about 24°C in the living room, I really don’t understand how anyone can tolerate 15°. You can at most sell/inherit the house to hardened loners or combat divers if that should ever come into question...

If I want to commission a good heating installation that feels very comfortable later on with the underfloor heating, do I then have to commission the lowest possible flow temperature?

How many sqm should one heating circuit cover at most? Or what does the number of planned heating circuits depend on?

What points must be considered in the offers from heating installers, or what are the essential points that matter?

Is the installation "according to calculation" sufficient?

Thank you.

PS: Concerns a gas boiler with underfloor heating, without solar – but with controlled residential ventilation; KFW 100 house

Layman's opinion:
You should discuss the supply temperature - the lower, the better (with gas please do not overestimate) - for this the heating load per room must be calculated - often critical are bathrooms or rooms with many windows (living area). Additional costs for the pipes are, in my opinion, rather negligible if you find a fair supplier.
It is less about the square meter area, but rather about the pressures or the length of the heating circuits - I seem to remember that one should pay attention to a certain balance here.
If you contract separately, make sure who takes care of the screed insulation. Have the details explained to you, such as the edge strip insulation - what is taped how?
Make sure the diameter of the pipes is not too small (higher pressure) - but also make sure you do not lead your heating engineer into new territory... in case of doubt, master craftsmen know quite well what they are doing.
From my own experience: do not overcomplicate - in the end, the eternal optimization saves 2 euros a month - so yes, if the calculation was done well, that is sufficient.

We were told that with a 17mm pipe, the maximum length of a heating circuit is 120 meters. Depending on the installation spacing, this corresponds to about 10m² per circuit...