Whatever dilemmas PHI might face during the course of this venture their staff presumably will be at arms-length to it all, meaning whether or not PHI get paid in a timely manner for the provision of their services should have no bearing on their staff getting paid on-time and in the correct amount.
Haven't PHI operated in KSA before? When I was working at Aramco 10+ years ago PHI always seemed to be on the radar in some context, whether they used to operate there at some point in the past or whether they just provided aircrew, I'm sure they're seasoned to know full-well what they might be up against there.
About the US citizenship requirement, presumably that's a PHI company thing. There's nothing preventing citizens of other countries working in KSA, unless of course you are Israeli!! In which case I imagine all staff on the 28/28 roster would be captured in the US taxation web in some way.
Haven't some Ppruners here have been offered a job there? Perhaps they could enlighten us on the question of tax??