There are a lot of questions here. I’ll answer the first one about BT, Kiril, and Wilbur.
- Boldtusk‘s special includes a 48% attack buff. Kiril’s special includes a 30% attack buff. These do not stack and will override each other.
- Kiril’s special includes a 30% defense buff. Wilbur’s special includes a 63% defense buff. These do not stack and will override each other.
- None of Wilbur’s other special skills have counterparts with Boldtusk or Kiril so they will not be overwritten and will not override others.
To dig deeper into specials and what stacks, may I suggest this thread.