Simple note - USC-28 and OM-55 of the late 70s for the DoD worked as a bent pipe repeater with a "T0" reference at the satellite slaved to its beacon. That allows TDMA and CDMA for multiple links. OLD OLD technology.
For the rest, for brevity here, you have a message here on the system.
And I note that you have generally good insights. I'm, hopefully, adding some ancient experience to the mix.
Addendum
I also note, existing systems (IRIDIUM and INMARSAT-M) are aimed at different markets so are not drop in solutions. However, their in-flight entertainment Internet links could be twisted into working nicely at lower volumes of data.
{^_-}