I did not see that there was a 3rd page to this thread so my last reply was actually a reply to #29, which had already been answered by David.
How could the pre-flashes ruin the sync only at 1/1 and 1/2 output?
In SU-4 mode pre flashes will
always trigger the the SB-800 before the shutter trips, throwing off the sync.
SU-4 mode works best when everything is in full manual mode with no pre flash events happening.
The reason you might no longer see it working above 1/2 power is that the flash could have recycled fast enough between the pre flashes and the shutter opening to fire again at the lower power settings. At the higher settings it takes longer for the flash to recycle and fire again.
If you haven't already I would suggest to set everything to full manual modes.
Put your E-M10 in manual exposure mode & set the shutter speed to 1/125s, iso to 100, aperture to f/8.
Make sure the flash settings on the E-M10 is in full manual mode as well, no TTL, no auto trigger. Set it to half power or a bit less.
Point the camera at the SB-800 with the SB-800's optical trigger pointing toward the lens, SB-800 set to SU-4 Manual setting.
Now try the test again, is the E-M10 syncing with the SB-800 through the entire power range?