Bunkermagnet
Journeyman Pro
You can possibly change this to a heat detector if it's causing nuisance alarms. Clear it with your insurance company first, but a working heat detector is much more use than a covered smoke detector.
He may not be able to. The laws and regulations on rented or accomodations in buildings of multiple occupancies may dictate otherwise. The fact the building was built in 2012 says it probably will have to be a smoke detector. The reason they probably covered it was because their oven grill is dirty and smokes heavily when they cook/grill.
As far as the cleaning of the property goes, as long as they aren't doing physical damage I don't see you can do much. I do assume you have a clause in the contract stating the flat has to be deep/professionally cleaned at the end of their tenancy?