For the carpet, for 6 years, even it is a new carpet from start, it need to be replaced any way. Same thing for the wall paint. House need to be re-painted for that long no matter who lives in the house. All the charges the landlord applied to you should be considered as "normal wear and tear", should be repaired and replaced by landlord.
They just trying to frame you guys for extra money. I do not think they can win the law suit if they take it to court. I would dispute the charges and not paying the bill.
For a 6 year tenants paying rent every month, come on, the landlord can find a better tenant than you guys. Talk to the landlord, tell them the $900 deposit is theirs, no more.