If you get a bootable OS on the external drive, you should be able to boot from that drive by holding down the option key at startup to select your startup disk.
I'd try creating an Win 10 install using either a PC or the technique I described here:
Make sure you do not activate Windows 10 until after you are booted from it and running it directly on your Mac. Otherwise it will be licensed to the virtual machine or other PC that you used to install it on the external hard drive.