I've tried resetting the defaults under Slew Mode. but nothing seems to work. I can edit the settings in the GSX dialogue, but cannot seem to move the markers at all in 3D editor mode.
If the other addon has taken the default Slew keys, it's doesn't matter if you reset them to default: it will still be able to "take" them, regardless to what they are mapped.
As I already explained in the referenced thread:
http://www.fsdreamteam.com/forum/index.php/topic,14266.msg105186.html#msg105186If another 3rd party program ALSO intercepts the default Slew events (rather than reading the key itself), changing them won't do anything, since both GSX and that other addon will then continue to compete in the usage of the same (now different) keys.
You simply cannot use them together. Of course, GSX does the correct thing, and it GIVES BACK the Slew key events to the sim, when you are done with the GSX editor. It doesn't "keep" the event for itself for more than it's necessary, and it's expected that all other addons will do the same.
For example, if this camera addon has a "global" hotkey that enable/disable the custom camera system, it should take the default Slew events only AFTER you pressed the key to enable it, and it should give them back to the sim when you pressed it again to disable it.
If it's not working like this, then you can expect it will conflict with lots of other products, not just GSX.