On the python questions, I have no idea.. seems backwards compatibility has taken a backseat in the Debian world and seems more a question for the developers of the installer
mtools is stock installed in VSIDO
I am not sure on the gpt question, if I understand it all you would have to wipe everything to go to a gpt partition table so if you have to do the same with an msdos table, why would you? As to if I am missing a msdos partition table, these machines and V-Box setup have always had the exact same msdos partition tables from the beginning...
The solution (create a msdos partition table while in LiveCD) seems as if that is the issue, but how is that possible if there always was a partition table and the April 2018 ISO's no doubt see the partitions and partition table
