Well, for starters, Bhai Gurdas ji wrote this, not me, and of course it is accepted as ਪਰਮਾਣਿਕ (authentic) by the Panth. Far be it for me to state the full and final meaning of any tuk, from any source.
But since you asked my opinion, yes, I would say all of the above. The stain remains in the sight of Guru Sahib, the sangat, non-Sikh, and the bemukh, too.
For a practical application of this, I would say: I would be highly skeptical of a adult pracharak who has full knowledge of what Guru ji wants us to do, and still commits bajjer kurehits, and then later does peshi, and wants to sit on the stage above the sangat and lecture them on Sikhi again. I think most of the sangat would agree.
So all of those "babas" in the Youtube videos posted by the OP, they would not have the right, I think, to sit on the Gurdwara stage and preach. If they want to pesh and live a humble Sikh life, that's fine.
A moorakh like me would need to listen to Katha on this. For now, what in your opinion, is the summary of what you posted? The blemish of apostasy remains in the eyes of who? Guru Sahib? The public? The apostate himself? I can understand that it remains but what exactly is it saying? Is this blemish one of distrust?
No apologies required.
Just to let you know where I'm coming from, Bhai Gurdas ji says when you become a bemukh, even if you come back to the Guru, the stain remains:
ਜੈਸੇ ਘਾਉ ਘਾਇਲ ਕੋ ਜਤਨ ਕੈ ਨੀਕੋ ਹੋਤ ਪੀਰ ਮਿਟਿ ਜਾਇਲੀਕ ਮਿਟਤ ਨ ਪੇਖੀਐ ।
Just as the wound gets cured with medicine and the pain also vanishes, but the scar of the wound is never seen to vanish.
ਜੈਸੇ ਫਾਟੇ ਅੰਬਰੋ ਸੀਆਇ ਪੁਨਿ ਓਢੀਅਤ ਨਾਗੋ ਤਉ ਨ ਹੋਇ ਤਊ ਥੇਗਰੀ ਪਰੇਖੀਐ ।
Just as a torn cloth stitched and worn does not bare the body but the seam of the stitch is visible and conspicuous.
ਜੈਸੇ ਟੂਟੈ ਬਾਸਨੁ ਸਵਾਰ ਦੇਤ ਹੈ ਠਠੇਰੋ ਗਿਰਤ ਨ ਪਾਨੀ ਪੈ ਗਠੀਲੋ ਭੇਖ ਭੇਖੀਐ ।
Just as a broken utensil is repaired by coppersmith and even the water does not leak from it, but it is repaired form stays.
ਤੈਸੇ ਗੁਰ ਚਰਨਿ ਬਿਮੁਖ ਦੁਖ ਦੇਖਿ ਪੁਨਿ ਸਰਨ ਗਹੇ ਪੁਨੀਤ ਪੈ ਕਲੰਕੁ ਲੇਖ ਲੇਖੀਐ ॥੪੧੯॥
Similarly, a disciple who has turned away from the holy feet of the True Guru comes back to the Guru's refuge when he feels the pain of his actions. Although he is freed of his sins and becomes pious, yet the .blemish of his apostasy remains. (419)
ਭਾਈ ਗੁਰਦਾਸ ਜੀ : ਕਬਿੱਤ ੪੧੯ ਪੰ. ੪