Insulation, subject of vapor barrier and holes

After all the connections/sockets/switches were installed, the blower door test was only then carried out with good values, after that the owner hardly lived in the house and did not install anything, so I assume that the ACTUAL condition regarding airtightness is OK. My question is whether there is a risk of ruining this ACTUAL condition with a few holes for mounting, e.g., curtain rods, etc.

No, you can do it without worry. You won't manage to make so many drillings that it causes any practically measurable difference. However, I don't find the gypsum fiber boards as a fastening surface very great. Theoretically everything works, practically I found the few we have here only conditionally suitable for heavier loads – otherwise we have OSB boards behind, and with those you can fasten almost anything. In your place, I would check where you might need cavity wall plugs and/or where you need to aim for the beam layer.