If they physically cannot move then they cannot move. Not much that can be done there.
If you can indeed show documented costs that you have incurred due to the sellers defaulting on the close of escrow date then you should be able to get an addendum signed that will charge them for the rent back and recoup you the dollars that you lost pushing the date out. If they do not agree to pay then you can try to recoup costs in court after escrow closes as well. Technically once escrow closes the home is yours and you can start eviction proceedings. However I think you will find that going that route is not going to help you much.
My advice would be to be cooperative with a firm understanding that the sellers will recoup your costs in one way or another.
