Sorry cannot advise you on the grass seed problem.
Do you know if the grass seed has gone in up the nose?
Why did the vet say it was dangerous?
Also how does the vet know it's a grass seed?
I think I would get a second opinion.
A few years ago one of my cats swallowed some grass. She was sneezing each day, a few days later she had a sneezing fit and the blade of grass came down and out through her nostril, my poor baby panicked a bit but she was fine.
Do you think it could be a blade of grass?

Hope this might be of use to you.
I have had second opinions with other vets. Your main vet shouldn't mind as it's your cats health that matters at the end of the day.

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