This would be considered a fixture to my knowledge,and unless specifically stated in the P&S agreement, the seller cannot take those, particularly if it were to leave a hole in the ground or some other destruction to the property. Your agent should have some firm words with the seller's agent.
One of the things a buyer's agent should have asked you when they were taking the contract with you was what is YOUR schedule? If the agent was not willing to make their availability coincide with your schedule, they should not have taken the contract. After all we work for YOU. We should be available for YOU when YOU need us. I don't feel like asking an agent to work after 4 is unreasonable at all.