Bummer. Not having a lot of luck with the xkb stuff.

From what I have found, it looks like Xephr is hardcoded only to look at event0 - event3 (from strings). I'll have to see if I can find the source code somewhere and verify that.

You can mostly fix the problem by changing the following lines in the n810 .Xmodmap-keymap file:
keysym plus = plus plus equal
to keysym plus = plus equal equal
keysym apostrophe = apostrophe apostrophe question
to keysym apostrophe = apostrophe quotedbl question

The plus/equals key will still be backwards on the USB keyboard, but at least you can type an = now (shift-+).