Your mum is right, you do not need to cancel the application.
They ask about the number of rooms other than a kitchen or bathroom in order to determine whether or not the UK property will be overcrowded with you living there too. You can’t sleep in a bathroom or kitchen, so they only count bedrooms and living rooms. So, with 2 rooms total (bedroom and living room) you can have a max of 3 people living in the property. With 3 rooms (I.e. 2 bedrooms and a living room) you can have a max of 5 people in the property.
Assuming it’s just you and your partner who will be living in the property, you only need 1 room (i.e. studio flat with 1 room which serves as both a bedroom and a living room), so whether you wrote 2 or 3 rooms won’t make a difference for the visa because you only need 1 room available to you. However, if it makes you feel better, you can correct it in pen when you print the application and/or make a note of the mistake in a cover letter.
You do not need to send your husband’s actual passport, just a copy of the bio page.
You must send proof of the accommodation where you will live together in the U.K. - if your husband owns the property, you need his Land Registry document (and ideally his latest original mortgage statement). If he rents, you need his original tenancy agreement and a letter from the landlord giving you permission to live there.
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