Nim ji,
I guess I should be clear that I am not big on sin in particular the kind that Christians, Muslims and Jews talk about. So:

Q#1 My answere is no.

Q#2 Is it a sin?, No but I would wonder about that person's sincerity but maybe the new path was theirs all along.

Q#3 Obviously everyones situation is unique but Lord Shiva knows what is in the heart.

Om shanti shanti Om
Satyaban

__________________
Om shanti shanti Om

Satyaban