Response to your questions:
1, i7 version of X220 does have a USB 3.0, but not a native one, so there
might be some compatibility issues.
2, As I said, starting Haswell, Intel implemented an anti-feature named "Boot
Guard". I believe that there might be very few models with Boot Guard not
activated. But it's very unlikely that you run into a computer without Boot
Guard.
3, AMD processors have a similar anti-feature named "PSP". So they are no
better then Intel ones.
4, Only the first generation X1 Carbon (based on 3rd generation Core) doesn't
have Boot Guard. If you see a Carbon with 4th-generation or newer CPU, run
away.
5, Personally, I don't like 14-inch ones...
