When I feel spiritually stuck and disconnected, I worry about Baba's physical absence. How does his nature as an avatar address this?

Your feelings are understandable, but reflecting on Baba's true nature can provide great comfort. Chapter 43 explains that great beings like Sai Baba take on various avatars with the sole desire for the welfare of their devotees. They are not touched by birth and death, which are considered false imaginations for them. Baba is described as a mass of bliss, the complete Supreme Brahman, who is devoid of bodily impulses. His actions were all part of a spontaneous play of Yoga for the sake of his devotees' salvation. He has not left, but merged into the unmanifest, and continues to keep his devotees spiritually awake from that state.