I'd like to do without commas, but 6 feels odd if it doesn't have commas.

She gave me a stiff smile.

Evidence that the above example is farfetched can be seen by comparing the total occurrences of component X.

It is not unnatural that you should feel anxious about your first flight.

The two of us don't belong here.

Do you feel anything unnatural when you put your teeth together?

It's very easy to sound natural in your own native language, and very easy to sound unnatural in your non-native language.

This sentence sounds unnatural to me as an American.

Generally, the sense whether or not an expression is natural can't really be relied on. For example, even if it feels unnatural, it is often easily due to lack of experience.

"What should we do with this unnatural English sentence of yours?" "Change it or delete it. Just make it work somehow."