Seems to me that a lump of ice does not arise on its own. The complete area surrounding the lump must also have been of the same density. Frost does not build up to this extent, in one area, unless it has not been taken care of - meaning defrosted as necessary. Partial defrosting does result in these type of lumps!
It is my opinion that the property should be returned to the landlord in good condition, and apart from fair wear and tear, the property should be as clean as it was when you first moved in.
There is no problem with wear on carpets, provided there are no large tears, burns or stains.
Reasonable wear for the time you were there is to be expected.
Other items like odd marks on walls is to be expected.
I am sorry I feel that under the beds should have been clean and dust free, the shower curtain should have been clean. Would you have rented it in the condition that you returned it to the landlord.
When you rent a property you should care for it as if it were your own.
Browtal