The easiest way to spoof the nvidia shield or something else is via:
I have these games favorite'ed:
reboot the mojo
make sure location access is on.
I installed it as follows:
Install the app from the link.
Force stop google store
Run the app,
Selected Tablet, Nvidia Shield
Pick your country and ISP
Logged into google for device registration.
Start google store and your favs should be purchasable.
It's a hit and miss process but it does work, all the games above were able to be installed expect GTA-4.
If it still fails force stop google store, rerun the market helper app and check.
if it still fails clear the data on both service and store and force stop google store, rerun makert helper.
On SW reboot and service/store data clear it remembered the settings for me.
Now you can pay for HL2 and portal but cant install them because they check to make sure it's a nvidia shield.
I'll have a look to see if there's a way around it.