Here's a quick initial sketch of the modification I'm planning to do to the original circuit (didn't add components values, refer to the original schematic)
the modification allows to use the unit in default mode (with both switches open) or to allow for different modes of operation; by closing SW1 (and connecting the PTT circuit), the TX antenna will always be the #0 since when the PTT powers the relay, the latter will cut-off power from the remote unit so selecting antenna port #0, while in RX one will be able to use any of the four antennas.
By closing SW2, the separate TX/RX port feature will be enabled so that it will be possible to use a separate receiver, the "RX mute" contacts could then be used to "mute" the receiver when transmitting.
I believe that with such a simple mods the switch could become even more flexible; another mod, if desired, could be picking up the relay power at the remote unit and use it (with a little bit of circuitry) to power a remote active antenna or RF preamplifier, such an option could only be used with SW1 closed and PTT connected, in such a case antennas #1, #2 and #3 will be RX only ones, while the #0 will be the one used for TX (and, if selected, RX)