Actually iff the protocol documentation is complete it is sufficient to implement Pactor 3, you do not need their source code (Just a C compiler, the protocol and some time to pen your own implementation).

Pactor 3 is documented in ITU-R 1798, which may be more useful then the SCS technical description.
Not really, it is also available to any ham willing to turn the protocol documentation into either software or hardware. Of course if the documentation is incomplete or if the real signals do not follow the documentation (quite possible, writing correct documentation is hard) then you are into the field known as 'reversing' and that really needs access to the hardware to generate known signals.

You do not need to have code identical to SCSs to have interoperability, you jut need to produce something on air that follows the protocol, after all we have probably got a dozen or so implementations of the TCP/IP protocols out there and they all talk to each other just fine.
Having a reference implementation is very helpful, no doubt, but it is not required.
Well you could do that, but it sounds a little painful, I would start with the ITU documentation and something like gnuradio (Provides convenient building blocks), and actually the interleaving and coding is described in the protocol, as is the higher level packetisation, so once you have the output from the viterbi decoder working the rest looks fairly straightforward.

73 Dan.

Pactor 3 proprietary source code is not available.  Only one source for P3 modems.  I see little difference between this and P3, yet the FCC looks the other way.

I can't find a good picture of the back panel of this radio, but I presume it requires 13.8 volts to power it?

The back of all the TS2000 radios are identical.

A line of description leads me to believe that the radio's second (sub) receiver cannot function in SSB/CW/Data Modes. Is that so?

That is true

Another line I read leads me to believe that the radio cannot transmit on VHF and receive on UHF (Or vice versa) simultaneously.  Is that also true? There is a line in the specifications about "crossband repeat" but I don't think that's applicable to working satellites with uplink in one band and downlink in another.

The radio definitely CAN receive on UHF and transmit on VHF simultaneously and vice versa.  Using the "Satellite" mode for doppler shift makes operating a snap!

This radio is purported to be one of the few transceivers that can perform Satellite Communications, but if the sub-receiver will only do AM/FM. that leaves out a lot of the possibilities for this field of ham radio.

If you are using one of the SSB birds, you use split mode to operate.  I have had numerous contacts on FO-29 this way

Thanks for some help and information from someone who has one or know the story here.