Yes, her memory loss, depression and possibly also a steep decline in cognitive function is very likely on the basis of chronic oxygen
deprivation when awake, and exaggerated oxygen
deprivation during sleep. And, low oxygen levels
can contribute significantly to heart failure and make it less responsive to therapy than it would otherwise be.
What you describe suggests that she should definitely be using a continuous positive airway pressure (CPAP) device for her sleep apnea.