Yo, BG, I aspire to be retired...some day!
Better to aspire or inspire than expire. LOL
It's very weird to be forced to give the answer 'Retired', which apparently fits me since I stopped 'work' at the age of 48. The next welcome blow to the ego is when you only have an age category of 'over 60' to choose, as if you're identical to a 95 year old in your behaviour. Just kidding - I love being retired and having time for rituals etc.
Seriously, though, the OP's question seems to have been answered in that the definition of a Ngagpa and what it entails differs quite a lot.
This is a bit like the threads where Ngondro has cropped up. Those whose Gurus have a view that to become a Ngagpa requires vows etc. may be upset that other Gurus don't. Those whose Gurus don't require those things may feel doubt that maybe they aren't doing it 'properly'.
That's the blessing and curse of an internet forum. It can expose us to teachings and Lamas appearing to contradict each other.
Sometimes this results in the response: 'That's interesting, my Lama teaches this differently. Let's share.'
At other times, people feel threatened and the response is (paraphrase): 'How dare you contradict my Lama who is an enlightened being.'
One thing I have learned is that 'one size fits all' can be a real problem. When Gurus simply have no time to give personal advice due to having thousands of followers and a busy schedule, people can end up doing things incorrectly or failing to recognise what is progress and what is wishful thinking. If it is left to senior disciples to give the guidance it can be a bit of a lottery or a very rewarding and enriching experience.
We must also remember that our qualifications and titles may reassure some people, but others don't care and want to judge your performance.
I've a bucketful of professional national prizes and titles but when you retire they are completely useless. It's a great lesson in preparing for death.
So is a Ngagpa someone who has mastery over mantras and has proven results or someone whose Guru has trained them and recognised them as such. Why not either or both? It doesn't matter, as we're surely not looking for universal acceptance in the way that a doctor may need to prove the transferability of their qualification when applying to work in another country.
I also doubt that any beneficiary of successful practice will care.