Tapeware has been replaced!
After all the trouble I went through with Yosemite Tapeware I finally abolished it. Looking around for some scripts around Windows' integrated backup (NTBackup) I found this (German) very good howto (English version here) by Denis Jedig (thanks for the great work, it saved me a couple of hours).
After I had collected the GUIDs for the devices, libraries and other items described it was just a matter of writing some wrapper scripts to manage service stops/starts before and after the backup. First full backup went through without any problems, sent me the expected report via mail and logged everything nicely.
I will now have a closer look at the log files of the incremental backups and behaviour when wrong or empty tapes are inserted as well as when they get full. I will keep you posted, in case anything interesting happens.