Christoph,
You're right, the main part of the clubhouse did burn in the 50's, but part of it still houses the mens & ladies changing rooms.
From your findings, the course obviously lived on and was extended to 18 holes.
I should have said Club and not Course, when saying it moved to St-Germain. The source I have states " la société du Golf de l'Ermitage, à Port-Marly, dont les status ont été déposés le 3 juillet 1903, écrit la lettre suivante au directeur général des Eaux et Forêts : "La Société de Golf et de Tennis de l'Ermitage (Port-Marly), dont j'ai l'honneur d'être le Président, n'a plus que deux ans de bail pour les terrains qu'elle occupe actuellement ; ledit bail ne sera pas renouvelé"
The date is not the same, but it's really not that important.
This information still doesn't answer who designed Port-Marly, either the original 9-hole or the extension, if it was just an extension ?
The mention of Louis refers to Louis Trouvet who was Secretary, Greenkeeper & teaching Pro at St-Germain. Maybe it was him that layed out the 9 hole at Port-Marly ?