Is it permissible for the congregants to pray on the upper floor, listening to the imam who is alone in the mosque's prayer hall, after the carpets have been washed?

The default is for the Imam and the congregants to be in the same place and at the same level. It is disliked for one of them to be higher than the other without need. However, if there is a need or benefit—such as cleaning the mosque carpets—then there is no harm in that, and your prayer is valid.