im pretty sure they do all stack. combat mastery items in game are typed as enhancement, insightful mastery are typed as insightful, and kta is a different kind of bonus (i just know it stacks

). its possible that they arent functioning properly in some way. when i am actually wearing a combat mastery and insightful combat mastery item at the same time (l animated rope, l warriors focus) ill test it out a bit. assuming they work like they should, they should certainly stack. i know kta does stack with insightful combat mastery and individual tactic dc bonuses from gear from past experience, at least.
insightful combat mastery can be found on consuming darkness, spare hand, emerald gaze, warriors focus, hook boots, dun robar rings, drow weapons, and possibly others im forgetting.
in my experience legendary ts/hox have easier dcs than the quests and shroud.
trip is much easier to land than attacks against fort.