Exceptions to the hearsay rule are deceptively simple on the surface, but very complex in practice.
I have no doubt the defense will fight strenuously to keep out A&K's testimony. My non-lawyer's assessment is that they will fail, due to the "admission of guilt" hearsay exception.
I would guess the defense will also try to get RM's testimony about what BM & KM told him admitted. My non-lawyer's assessment is that they will fail, due to that testimony not falling under any of the hearsay exception rules.
They'll get Bob on the stand, no doubt. He'll testify about where he was when he heard about the shooting, and when he first got home after the shooting. He'll probably testify about any previous contact he had with EN, or times that he saw EN with TM or any of his kids. But I doubt they'll be allowed to ask him what BM & KM told him about what happened. Again, that's just my non-lawyer's assessment.
Bob might actually be smarter than what we give him credit for. He's kept the kids under wraps; they've hardly said anything publicly that they could be impeached for on the stand. Bob can't testify about what they've said to him, so he's free to say whatever he wants publicly, and it's likely that none of it will be admissible in court.