I am familiar with what you describe Tibetan_Ice. Indeed leading up to nirvikalpa samadhi is a death like experience. You're on the right track. Keep going.
I was a Hindu monk for a few years and I'm glad you found in your research what Sivaya Subramuniyaswami said about nirvikalpa samadhi. He also says a few more things about it in a different section of the book that I think you would find interesting.
Here's a link and the quote.
"Many people think of the realization of timeless, formless, spaceless Parasiva, nirvikalpa samadhi, as the most blissful of all blissful states, the opening of the heavens, the descent of the Gods, as a moment of supreme, sublime joyousness; whereas I have found it to be more like cut glass, diamond-dust darshan, a psychic surgery, not a blissful experience at all, but really a kind of near-death experience resulting in total transformation. The bliss that is often taught as a final attainment is actually another attainment, Satchidananda, an aftermath of nirvikalpa samadhi, and a "before-math." This means that Satchidananda, savikalpa samadhi, may be attained early on by souls pure in heart. It also means that one need not gauge the highest attainment on the basis of bliss, which it transcends."
There is a bit more on that page if you're interested.
When you begin to touch into this, as you are. Coming to the brink of nirvikalpa. There can be many physical symptoms that come along with it as an aftermath. Of course these may not apply to you, since everyone is different. And over the course of my spiritual practices I have experienced many bizarre physical symptoms after coming out of deep meditation. Most likely the burning up of certain vasanas. I get bad headaches and my body has a difficult time getting warm. My hands and feet become ice cold. No hot showers or warm food or heating warms me up. It took 2.5 hours the other day to "return to normal".
The reason for this is because as you meditate and your going into nirvikalpa. The energy/heat within your body begins to withdraw into the top of the head. Until eventually one is absorbed and the energy completely leaves through the top of ones head.
I was taught this as a monk to withdraw this energy consciously but it happens naturally when someone is going deeper. I observe this in my meditations and can feel my extremeties becoming colder and colder. Then I come to a point where my sense of self begins to feel like it is dissolving. Not in a blissful this is wonderful way how most people think of it. Because in order to experience bliss there must be an experiencer. Nirvikalpa is when the experiencer is dissolved temporarily. When someone first begins to touch into this but not fully absorbed. There can be a degree of fear that arises. You realize this is a lot more real than I thought, ha. Your sense of self no longer has any footing. Nothing to grab on to and feel safe and secure. Even the mind uses the higher experiences of bliss to identify and hold onto. It is quite difficult for most to go beyond these wonderful experiences. I have noticed that the ego uses everything in its power to stop you from going any deeper. But everytime you go back to that edge, there will be less and less fear until one day. Awareness disappears. No more identity, no mind, no universe, no earth, no people, no higher energies, no deities, no bliss, no experience.