I will tell my personal story about this. To begin, accepting the blessing of Amrit is a huge step, bigger even than marriage. Marriage ends at death; Amrit is forever. At the risk of offending some, I see Amrit as partially a marriage betrothal to Akaal Purakh and also an adoption as a child of Guru Gobind Singh ji. It is a huge and very serious step. It is a great joy and a great responsibility. To break a sacred oath is a big deal, and the promises one gives at Amrit are the biggest of all.
To break those promises is a big deal,most especially the four kurahits. I know. I did exactly that. I broke three of four of them. (No prizes for guess which three.) On purpose. Eyes wide open. I planned to break all four, but just couldn't...
Perhaps I was insane. It was 1985 and I was still reeling from my personal losses of 1984. I felt sorry for myself (ego). I was angry, very angry. No one dared talk to me about hukam or destiny (I believe in both). I was especially angry at Guru Gobind Singh ji. He could have prevented all this if he had wanted to! I forgot or disregarded his own great sacrifices. I threw a huge temper tantrum. I told Guru ji to go away and leave me alone. I cut myself off from the Khalsa and the Saadh Sangat and disappeared into the night.
The next 20 years are partly fog, partly learning experience. I left my family and my country. I changed my name. I cut my hair - only once, but that made my point. There was nothing to identify me as Sikh. Still, once a while something would happen that would remind me of who I had been and what I had become, a dethrowned princess who had denounced her royal lineage.
Guru Gobind Singh ji is a gentleman. Although little reminders were dropped in my path - an elderly Singh ignored by his family who needed my help, a woman Christian missionary named Mona - he never intruded in my life. I know now, though, he was always there, patiently waiting.
One day, something inside of me wanted badly to go back home, but I was afraid. I had made a complete mess of my life and had no idea how I would be received. And I was far too ashamed to face Guru ji in the form of the Panj Piyare. One day, I found myself wandering into a gurudwara and, without being quite sure how I got there I was sitting, cross-legged, all alone in the darbar sahib. Just sitting. Just me and Guru ji. I suppose the granthi was lurking somewhere. After a time, curiosity got the better of him, wondering who was this gori in the gurudwara. (I am half Punjabi, but I look like a gori.)
I ended up telling him my story. To cut the story short, a short time later, I was blessed with Amrit for a second time, whole-heartedly welcomed back, no recriminations.
I once had a son. There was nothing, absolutely nothing, he could have done that would have lessened my love for him. How dare anyone imagine that Guru Gobind Singh ji is a less loving parent than I am!
This is getting too long. I'll conclude by saying that if you break your Khalsa promises, it is very sad and a big deal. Really it is. You have, by your own action, cut yourself off from the Khalsa. However, Guru ji's kirpaa is a fact and you can return any time you choose. It's a bit scary and very hard on the ego to present yourself to the Panj Piyare, and when you do, the forgiveness is total. I strongly suggested it to anyone who has put themselves in this position. [Note aside: any messes you have made in the physical universe are still there to be cleaned up; they do not magically disappear. I, at least, have been given the strength and power to do this in my life and I believe you would, too.]
"Being lost is worth the coming home."
(As for the Nihungs, the have their own Rehat Maryada(s) and do not follow the SRM. Far be it from me to pass judgment on them.)