Install click vinyl despite possibly too high residual moisture.

Should we maybe ask the questioner first whether there are actually measuring points marked in the screed? Not an insignificant aspect, so as not to hit the heating pipes with the hammer and chisel during sampling ...

I assumed that a screed layer always does that.

Can't you just use a corner of the room or the area in front of the entrance door? For me, there's easily 30cm of "air" between the door and the first "snake."

But... if it's anyway inevitable to lay the floor and move in because the apartment has been terminated and won't be extended (the next tenant probably wants to move in from 1.1?), let alone paid for... then you can save yourself all the hassle with the measurement. Because what will you achieve?

At best, you can lay the floor and sleep peacefully...
At worst, you're right on the limit and don't know if that's only the case in one room or everywhere. Then you lay the floor and sleep restlessly...
In the worst case, the measurement is clearly above the limit, you still have to lay the floor, and hardly sleep at all for the first few weeks...

Lay the floor everywhere you still can. The rooms are still empty and it's easy to do now. Once stuff is standing around and you have to move everything back and forth for every little thing, with kids running around in between, you hardly get anything done anymore.

The following issue:

The issue is there because no one ever says that it's time to bring one or two dehumidifiers into the house! One week should be enough.

Thank you for all the updates and sorry for the late response. The foil test looks okay, with only mini-minimal moisture visible so far. I haven’t placed a hygrometer underneath. Also, all materials (tiles, foils, cardboard, etc.) that are now on the screed are completely dry from below. We have literally left no stone unturned. We will continue heating properly for just under a week before starting the flooring work.

We also considered CM measurement, but no matter the outcome, we have no choice and must move in anyway. We are currently trying to organize construction dryers, but in the meantime I have procured two (30L/24h) consumer dehumidifiers, which we will also keep running and integrate into our lives afterward. They almost reach the dehumidification performance of construction dryers. For now, we are also leaving out the baseboards to minimize risk and be able to react quickly in case of mold.
I also asked our site manager (from the general contractor, who is in my opinion very good) how he would assess the situation. He cannot and does not want to give an official statement, but told me in confidence that in his experience he does not see a problem.

We also know that we need to heat a lot at the beginning and ventilate intensely. I follow the forum here very actively and absorb all your information on correct heating and ventilation behavior. Many thanks to everyone for that.

If the worst-case scenario is a new floor covering, then economically at least it’s not a total loss. Of course very crappy, but survivable. I don’t worry too much about the kitchen, since it will be in the driest room on the south side. Most moisture has always appeared in recent months in the upper floor on the north side. Logically.
Additionally, luckily we planned the floor plan so that no large furniture is against the exterior wall. The kitchen unit and the cabinet only have their narrow end to an exterior wall.

Fortunately, we don’t have children yet, meaning the two children’s rooms on the upper floor (north side) will remain empty for now and we will lay the flooring there only after Christmas. On the north side there is also my office (though on the ground floor) and the tiled laundry room + children’s bathroom. Tiles are not so critical there anyway.

Once again, many thanks for your assessment and for keeping things realistic. It’s not always like that here ;)

If you have any tips on how we can further minimize the risk, please share. We would really appreciate it.

The manufacturer, Windmöller GmbH, states the following in their installation instructions for "Wineo 600 rigid": "Deviating from DIN 18560, we recommend a maximum residual moisture ≤ 0.3 %CM for CA heated screed." DIN 18560 Part 2 specifies a maximum residual moisture of ≤0.5 CM-% for CAF. This means that the carrier board of the design floor covering reacts to (rising moisture from the back).

In this context, I also inquired with Windmöller whether a vapor barrier would make sense. The answer was that the Rigid 600 consists of a polymer material and a vapor barrier is only necessary for wood floors or floor coverings containing wood (such as laminate flooring or our multi-layer). Since our rigid vinyl flooring is not a wood product, an additional vapor barrier is not required. Above and below the rigid carrier board there is a flexible upper and lower layer made of vinyl, which forms the soft outer shell to the hard and firm core.

So everything is made of plastic, if I read that correctly.

(...)
Fortunately, we don't have children yet, so the two children's rooms on the upper floor (north side) will remain empty for now and we will only lay the floor there after Christmas. On the north side, there is also my study (however on the ground floor) and the tiled utility room + children's bathroom. But tiles are not so critical there.
(...)

Oh, I see. Then actually just do the necessary rooms and the rest after moving in. Good luck with that! I would probably risk installing the baseboards already... you tend to underestimate the effort and once the furniture is standing, no one wants to move it a meter away from the wall to install baseboards behind it. Keep a 10-15mm expansion joint, then nothing should happen?