the landlord is trying to fleece you.
You should not accept any further changes of contract. As the current contract is for 15.4. you can recover your expenses of not being able to move in that day and recovering the deposit.
The first thing would be to make clear in written (make sure to be able to proof reception of the landlord) that you do not accept to move into the department as it is and will recover the expenses. Normally you'll have to give the landlord some time to resolve the problems in order to be able to terminate the contract.
Actually it was the worst idea to accept the new contract from April 15 th.
Refer to the prior negotiations and declare that you terminate the contract without further notice (just in case you need to argue later about the termination).
The landlord is not entitled to ask for a deposit higher than 3 cold rents!
In any case you can recover part of the deposit even if you would like to continue the contract.
Having seen the conduct of the landlord I strongly recommend to attend a local lawyer at once. You should about the possible worst case costs beforehand. However, it should be possible to recover the lawyer costs from the landlord as well.