If you can get the valve bodies out, there are O-ring seals availible to repair them. If they're really stuck, I believe the 'sparkplug' trick should be used to pull, not rotate them. While I was replacing the heater lever assy on a friend's 72 280SE, with the radio and ashtray out, I could see the valves but they weren't easy to reach, though his car did have AC.
Short-term, the safest option would be to bypass the heater before your trip.

Happy Motoring, Mark

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DrDKW