Hello David,
- probably, you were not asked about grub during installation because you did not chose the expert installation mode
- possibly, your problems originate, at least in part, from the fact that albeit creating a new ESP (EFI) partition on your USB key, the installer try to use also the ESP partition(s) of the disk(s) on the computer used for the installation. I would suggest to verify that none of the partitions on the disks in the computer (ESP, swap, /boot, etc...) are used during partitionning.