So diving deeper it seems that zdb -R does not work on encrypted dataset. zdb -O was not working at all, so I looked at zdb.c to see if I could hack something in to bring my data back.
And a couple of hours looking at the code I found out that using the -dddddd switch and adding a line to dump a file to the path I have now a working zdb that writes a dataset objectid to the according file.
I'm now in the process of iterating over all files in that encrypted broken dataset and it seems to get me out the data.
Even though I'm quite dissapointed that zfs failed with raidz2 and just one disk down but I'm quite happy that zfs is opensource.