Yissera & Backgrounds Question

Hey there. I happen to have a 5e Yissera cert (shhh … don’t tell anyone). And I was wondering about how they interact with backgrounds that have a racial or national prerequisites. For example the Highborn background requires you to be human, kit or Val. While Legionaire & Royal Marine have national backgrounds.

Can I take any of those?

The fluff text of the Yissera cert & the Abessios entry in the Codex both assert that female Yissera are functionally Highborn. It seems to me that Yissera should have access to that Background even before considering infiltration.

On infiltration: What about playing a Yissera character that has been a long term plant within one of the militaries of the Known World? Can I play a Yissera who has imitated a Coryani long enough to have joined and served in the Legions? Or the Royal Marines of Milandir? Or are all such concepts banned because your nation of origin is Abessios?

Any official help would be appreciated.

(I know I have the option to swap out the feature of one background and replace it with another … so the character could potentially “emulate” any of the above backgrounds.)