Guess I'll post my first little achievement of the day/weekend here. I couldn't find much about this particular case, where you have logical live usb partitions on an extended partition which all need to be booted using grub. An easy solution is installing grub on the root of the hard drive in a seperate partition.
thread
This external usb HDD is, from now on, my insta-rescue-linuxinstall-data-buddy
