My question involves a security deposit in the State of: Montana
Last June, I signed a one-year lease with two roommates at $900 a month with a $900 security deposit. In March one roommate, unhappy with the fact that the other two of us wouldn't allow her boyfriend to move in, went to the landlord and asked to be taken off the lease. This was done on good terms, with her fulfilling her rent obligation through this June. Her key was turned in in March, she hasn't lived with us since then, and the terms we left on were at least neutral.
Now our landlord is saying that a new lease must be signed because she moved out, which seems reasonable. The problem lies in the security deposit - I paid it, and have the record of the check to prove it, but the landlord wants to do a walkthrough, refund the security deposit in equal thirds to each tenant, and get a new $900 deposit from the remaining two tenants. This seems largely unnecessary to me from their perspective and odd given that, again, the entire deposit was made by me from my checking account.
Our experience with the landlord property management company has been largely unprofessional - the apartment was to be cleaned before we moved in, which was never done; we were sent an eviction notice in October for a previous tenant; another previous tenant was a violent sex offender, which we learned after both his parents and an officer stopped by looking for him; the place was advertised as 3 bedroom when it is in fact 2; and they never required a wear and tear checklist to be signed when we moved in but have now produced a handwritten copy of looseleaf paper from the week before we moved in. We recorded video of the dirt and and damage when we moved in, so I imagine we're covered there. I bring these up because this current predicament seems unprofessional as well, do they really have any legal grounds for their request?
To summarize, is it reasonable to require a new lease and deposit rather than just continue the old on a month-to-month basis? Is their checklist valid without our signatures or evaluation? I just want to continue our current agreement and I'd rather not have my deposit refunded to someone else.

