This seems to be a situation where you need to seek help from a higher authority than this particular preceptor. Is there a possibility you can do this?
I would attempt to speak to this "watcher" and have her verify what she told you about this neb tx. I would also present, just as you did here, all of the skills and treatments you have mastered to this higher authority and attempt to have them interceed for you to continue with this hospital.
It is sad but true that preceptors often are overburdened when training other staff . I am sure you will remember in the future to check for documentation of care and treatments rather than take anyone's word they were done. A NSS neb is not life threatening and could have been repeated if there was any doubt it was done. I do not mean to lecture, just make an observation.
I will pray for you to have a positive outcome to this problem. I hope He showers you with blessings. Have a good day.