I use Kort's also, and am nearly positive they show it capping at 10%, at least in the version I have. It does cap at 10%, so if it shows 20%, it is wrong.
You can even see for yourself when you type /bonuses in game when wearing 20%, it will only show you as 10%.
The two exceptions to this rule are necromancers and animists. The animist bombers, which already have the highest delve in game, have a 25% cap. So do necromancer pets. The reason for this is b/c they are both treated as pets, and although Mythic kind of acknowledges it is not correct, has not fixed it yet. Pets are always different than players.
Well, if you are botted, a warrior should be doing great damage. You may be having problems lag and people who exploit this. I do not play melee toons, so I would be poor in a 1 vs. 1 against an experienced player b/c I would not do run throughs and lag expoits to get side styles off. Bonedancers are great interrupters, and if you can survive the initial hits and kite a bit, they can do solid damage as well. I still don't think it is a great duo combination b/c of lack of speed, but I guess it depends on where you are going. If you are at Bledmeer Bridge most of the time, speed is not much of an issue. Just keep playing together and trying to learn and gain bounty points to eventually get your master levels up. You will love be bodyguarded by your brother's warrior if and when he gets this ability. The new quests supposedly give out RP's and BP's for completing (this ghost keep thing), so that may be a way for you to get some bounty points to spend on Master Levels.
It sounds to me like you have a fairly good understanding of the game, so it probably is just an experience thing where you have to learn what works and what doesn't work.