I found new information about the abbey's archive. There were about 80,000 documents and books on the Michaelsberg, and most of them came to the archbishop's archive in Cologne (some to Rome) after the friar's departure. Before, they edited a "find book" with a table of contents which was ready in April 2012 and handed out to the archive managers of Siegburg and the county. The documents contain also information about the abbey's time as hospital, and the archbishops archive "is open for professional and hobby historians", and: The "find book" shall be published online in the internet!
I visited the Michaelsberg today, and the abbey's cemetary is only 50 m away from the rose garden, definitely not the one we are looking for. I was also one the new cemetary, but as written before only graves of russian and political victims. There is another cemetary in Siegburg-Kaldauen which have not visited until now, but there is a sign "Military cemetary", I'll have to go there and look, but this one is some kilometers away and not just "down that path".
Here three photos of the rose garden today evening: High walls on one side, deep walls on the other two sides and only a small entrance, it was easy to build a guarded camp here.
Christoph