[nomonnai]

As a German speaker, the claim "more than one prenominal genitive is impossible" seems interesting but perhaps not entirely accurate. As a non-linguist I probably misunderstand the meaning of prenominal genitive and might be missing your argument's point. For the overarching discussion we should note though the great variety of refering to "more than one element" in German.

> Anna's mother's sister's dog's house

> Das Haus des Hundes der Schwester der Mutter von Anna.

It's difficult to parse though. We only get to know about Anna at the end of the sentence. Consequently, we avoid such sentences or use workarounds.

> Von Annas Mutters Schwester dem Hund sein Haus.

If you ever use such a construction German speakers will correct your bad language but they will perfectly understand the sentence's meaning.

A quick search brought me to this presentation: https://www.linguistik.hu-berlin.de/de/staff/amyp/downloads/...

The take-away seems to be: "German does not work like English" (slide 8) so be careful when comparing PreGen in German and English.