When Murbella became the head of the BG-HMs, Idaho decided that he had to leave. There was significant buildup to his decision. As she was undergoing BG training, he agonized over the growing distance between them, and at the obvious changes that her training was bringing about in her. So when he left, he left Murbella behind, because of the rift that her training had created between them. He didn't want to be her pawn.
The posts about the progeny of himself and Sheeana are speculation. (However, they are highly informed speculation, an "educated guess" if you will.) Even if Sheeana is that young (I can't remember), that certainly didn't stop Frank in any of the other novels. And he did like to breed Duncan to the Atreides.
