Does dunstan truely love mrs. dempster, or are his feelings based on guilt and a sense of obligation?

This all depends on what point in the novel we are considering. Mary Dempster is a kind lady, a shameless adulterer, a mentally unbalanced introvert, and for Dustan, a saint. Though viewed through several lenses, she remains something of an enigma. Dunstan feels tied to her through his guilt over the snowball which Percy threw and which hit her, leading to the premature birth of her child Paul. Dunstan believes her capable of miracles, and marks three - bringing his brother Willie back to life, saving him on the battlefield during WWI, and changing the life of the tramp. Throughout his life, she remains a powerful presence in his psyche. In Mary's later years Dunstan feels more of a duty to Mary rather than captivated by her. She blames Dunstan for the estrangement from her son Paul. Dunstan tries to reconcile his once beautiful saint beatified by himself, with the broken woman who cannot break out of her prison of regret and anger.