It's a little bit of both. You have a copy of the rent that you did pay and that is confirmed in the emails.

You need to have yourself taken off the lease if possible. Try to make an arrangement with the landlord. If they are amenable to it, get in writing how much your room was so you know how much to charge your subletter if you are allowed to get one and to inform the LL what is going on.

They should not have the ability to change the agreement, but when you all sign a lease together, you are all responsible for the total amount of the rent. Didn't the LL question why everyone was all of a sudden paying different amounts than they were previously?