1
It is best not to use another device to get the new Windows 10 licence, because if you use the built option on the activation page, that licence will get associated to the device you are using, not the one you want to upgrade to Windows 10 on.
2
When you install Windows 10 using a bootable USB, you just select 'Skip this Step' so you can complete the Windows installation without a product key, you can then later add the product key from inside Windows to activate it.
3
Microsoftkeys.net is selling very cheap licences, they are most likely volume licences, not consumer licences, you should avoid that type of licence.
The licences on mychoicesoftware.com also seem to be too cheap to be consumer licences.