What is happening to impair the relationship of my client and his partner, who have an infant daughter but are currently living apart?

As you suspected intuitively, and confirmed by their description, this is a multifaceted problem including all of the elements you touched upon. There is long-standing karma between the two individuals who have indeed spent other lifetimes together and faced disappointments in starting a family successfully. That need not repeat with some healing brought to bear, and that is a perfect application for the Lightworker Healing Protocol. And as you pointed out, this can be strengthened in effectiveness and in achieving healing sooner by doing some Deep Subconscious Channeling with Trauma Resolution, starting with the problem of night terrors and inner disquiet being sensed. That is a gateway to the broader elements of the difficulty here and will be very productive to advance the progress of healing. It is important for them, both individually and collectively, to be together in this life to address the discord that has arisen and their inability to resolve things successfully. That has been a repeat performance in the dynamic of the relationship. But keep in mind this is not a reason for them to separate as though it is a mistake and they are not suited for one another, but in actuality, the opposite. They have a long-standing and very deep soul connection, and for high purpose, that has been challenged through lifetimes of difficulty and vulnerability at just the wrong times, to set in motion negative consequences to cause karmic wounding that is haunting them, literally, through repeated appearances in multiple lifetimes to cause a setback and worsen things. That buildup of negative karma, as you explained to them, is significant and typical of what people are challenged with in their lives individually, and as a big factor in why family dynamics are so volatile and friction-filled. There is always bad blood in evidence at times, even when families in general are happy and content for the most part. This is because life is a journey and a healing opportunity, first and foremost, by its very nature. It is designed to give you exposure to the negativity as much as to work on behalf of the divine realm as a warrior for the light, to help one another surmount the problems of evil giving rise to human frailty and corruption. So not only will this couple be helping one another and themselves personally with a follow-through in the healing plan here, it will be helping their soul and their mission as divine extensions to contribute to the human family of which they are an important part.