If the housing agreement gives them the right to search it doesn't seem like it is going to matter if it was in plain sight or not. If they had a right to come in and search the judge will probably allow everything from that point on. You might find some mistake but it sounds like an uphill battle. If they had a right to search but shouldn't have forced you to show them things then you might have an agrument but you might still face inevitable discovery, that they had the right to search and had you not shown them everything they would have found it eventually.

The Miranda rights normally wouldn't be that big of a deal as they don't have to read them to you until they start questioning you, not when they arrest you. However, I would consider that when they called you and told you to leave work or else they will remove you that once you arrived and met with the police you were in custody. Problem is that came from your roommate not them so good luck with that testimony. See, you can be in custody without being under arrest and some judges want the rights read then. You could say that since your freedom to come and go as you please was now under police control (a basic definition of custody), you should have been read your rights before they made you show them the things and incriminate yourself. Remember this phrase "In my mind I felt I was not free to leave and that I was under their control."

You may have some chances to toss the search but it's going to be hard and they still might get it in as inevitable discovery.