The Naths and Hatha Yoga
The short story is that the Naths and Hatha Yoga as propounded by Yogi Swatmarama only overlap on certain points. Certainly asana and removing obstruction from the nadis is important, but this is only a comfortable sitting posture for most Naths.
The focus on a Nath adept, male or female or devotion and concentration on a particular Siddha being or Guru was and is an approach by which one is seeking transference of the Shakti and knowledge of that being into oneself and life stream. This is a feature of many of the Siddha traditions.
The 64 Yoginis are Shaktis. Some people worship them in the same manner as above. In a yoga to absorb their potency.
With respect to the physical immortality story, Kaya Siddhi, while many yogis lived an extended span, most do not and did not. There is no evidence of human physical immortality and never has been. It is a myth. The real idea behind the concept is related to That which does not decay, the Spirit aspect of the human being. The concept of immortality in the Nath sect has always been a spiritual concept. A true Nath or Siddha is indeed immortal, though they may no longer inhabit a body, having cast it off at death.
It is this spirit continuum of the living and departed which is engaged as a part of the Nath yoga and tradition.
Warm Regards,