So in case that magnetic north is 2° east of geographic north, I have to correct this in therion by -2 and in survex by +2.
I am not sure with this. If you measure 0, true bearing would be 2 (assuming east is 90°), therefore I think you should use 2 in therion and -2 in survex. I assume, your situation is the same, as on this picture http://en.wikipedia.org/wiki/Magnetic_declination. OK, I messed this up. You are correct.
Question 2: When I enter the measurement data in therion, the meridian convergence is handled automatically (in case a "cs" is specified). However due to historic reasons most of our data is still in survex format. I import the ".3d" file and specify the "cs" during import. I assume that therion is not applying any corrections for the meridian convergence and I need to correct the bearings manually in survex? It is probably like that. If you want to have precise coordinates in your .3d file according to your CS, all bearing should be corrected for meridian convergence. But I am not sure, if you should add or substract it from magnetic declination :)
This, depends if you are east or west of the main meridian of the zone in the coordinate system. I think I understand the system now, but I'll need to draw some figures to explain it. Then I'll put it into the wiki. Best regards Wolfgang