From my experience you can just install windows 10 and use the win 7 key to activate it. I did it on my dell vostro laptop i got a month ago from ebay activated with no issue. as for the bios on legacy i would use a ntfs formatted drive high success rate from my experience
As to the Windows 7 issue if you would like to try and fix it. i just use diskpart in windows to make a bootable flash drive. open cmd in admin type diskpart when that opens. type list disk
type Sel disk and the number given to the flash drive example if the drive is disk 4 then you type sel disk 4 push enter.
Type Clean (all data and partitions are gone)
Type Create Partition Primary
Then type sel partition 1 type active. Finally type format fs= what ever file system you want ntfs,exefat ect add quick at the end for quick format.
Then just copy the files for windows 7 to the drive assuming you extracted them from the iso in to a folder.