Maybe I didn't explain it properly. The left side of the table is the DIFFERENCE in valour between your assassin and the target emissary. So if you have a +5 assassin and an +2 emissary, the effective level is +3, so 93%. If he is attacking a valour +2 princess instead, you add +3 to her valour, i.e. get +6, the difference is now -1, so a 76% chance of success. Attacking a level 6 leader (effective +12) would bring the difference to -7, so only 14% chance. Hope this is a bit clearer.

Nelson, as far as I know, yes, but this is hard to check due to the way the program handles random numbers.