but item bonuses WILL NOT STACK unless they are an artifact or a relics bonuses for example if i have "corsair" and "harecks leather" (both artifact/relics) the disarm skills from each (+5) will give me a plus 10 bonus and the highest other item of disarming (saying its a +12 belt) will give u a disarm donus of 22.however if you have two angels rings of disarming (with +5 disarm) and the same belt your overall bonus will only b +12 as the to normal rings don't stack with the belt.
There is a skill limit at 60 in some games and 64 in others and to proceed beyond this level will give you a minus bonus which cannot be counter-acted with the use of other items (even artifacts or relics)
In your game however i believe it may b the first item you attached is the one you draw power from (the +6 item) which will stack with the NPC to give you a bonus of + 12 to increase this to +15 i would take off everything bar the +9 item and keep the NPC whic MAY or MAY NOT stack I am not entirely sure.
Hope I have made this clearer for you!
Rhys