No, no one knows what it is. It’s reserved for special souls at high spiritual degrees. And a person can’t tolerate ordinary shame either. Everything one does is to avoid it. Otherwise one would live much differently.
The Gmar Tikkun is the final correction but after that, more happens. Only perhaps the greatest Kabbalists have had a glimpse of what. So, I don’t know about the ultimate fulfillment. But Eyn Sof means no limit, meaning according to the size of the vessel, you can fill it with no limit.
Yes, there’s a correlation between lines and Sefirot.
Many books talk about the written contents of what’s in Kabbalistic texts. We only consider, for purposes of rising from our world to the spiritual world while we’re still alive, the Zohar, the writings of ARI, Baal HaSulam, and Rabash. The rest is philosophy. We’re practitioners.