I don't know what "EU law change" this might be. It's probably more to do with the rebuild of Schiphol. Currently, you could probably happily access the Mercure, bringing your hold luggage (as long as you explain your intention to return landside again, to complete check-in of the luggage - though you may encounter resistance from staff along the way), as there is no security check until you reach the gate (you don't want to pass a hand-luggage-security check with hold luggage. The staff would be p*ssed, and all your liquids would be confiscated). But, from June, there will be a security check at the point where you go landside - it will no longer occur at the gate. So, getting your luggage through the security check is certainly far from ideal. They would probably refuse point-blank to screen your hold luggage, no matter what explanation you give them.

There are lockers at the airport where you could leave your bags whily you nip airside to use the Mercure.

But it's kind of strange to be dead-set on staying airside when you will have to come landside anyway to complete check in again.

There are plenty of hotels at the airport (Sheraton; Hilton; Citizen M) that would probably also give you a day rate - you don't even have to resort to one of the off-site airport hotels that would require a shuttle to reach. The Mercure certainly is not your only option.