Can someone recommend a clean and reasonable hotel at MXP for one night before our departure flight?

Look at Hotel First.

IF the point is that you will be turning in a rental car and really need that extra hour of sleep, then look at the Sheraton, which is right in the airport itself, so you can walk to check-in. If you are thinking of using a free airport shuttle, make sure any hotel you book that advertises a shuttle gets good reviews for reliability, short trip and operating the service in the early morning hours you need it.

If you won't have a car, it can be quite difficult to get to the other "airport" hotels because they are miles from the airport, and there is no public transportation to them and taxis are very expensive. Another reason most people don't prefer to stay at Malpensa is that it is dreary and the food is bad to non-existent -- a poor close to a fun trip.

But Milan and the several towns along the shore of Lago Maggiore offer great early morning transportation to the airport -- between 35 and 50 minutes -- plus great eating options and a chance to experience yet more of Italy.

I have friends who always stay at airport hotels because they just like to travel that way and I can understand that. But most people would rather stay someplace more interesting, even if they have a car rental to deal with).

We like the Villa Malpensa, which is an old fashioned place with a good restaurant, about 15 minutes from the airport.

We also like the Hotel First. They have a shuttle service, so we usually drop our car off the evening before, having dropped our bags at the hotel. Use the shuttle in the morning. There is a decent restaurant adjacent to the hotel, as well as one in the hotel, tho we have never eaten there. Early morning flights from MXP are the only reason we stay there, but what can you do?

Many hotels around the airport have shuttles. There are furthermore public buses and trains from Gallarate railway station and Busto FNM railway station.
BTW: a taxi from Gallarate doesn't cost a fortune

What other people do is drop off their car at Linate airport, take the shuttle into Milan, and spend the night in Milan, or they continue on by train to Lago Maggiore and spend their last night with a lake view. (You can also drop off the car at Malpensa and take a bus to the lake). All those options have regular reliable public transportation to Malpensa that starts at 5am and takes less than an hour. Still others keep the rental car to drive themselves to the airport before their flight, and there are many very attractive spots for a last night in Italy within reach of Malpensa depending on one's interests.

Just some options for people looking for them. Other people wouldn't dream of ever staying anywhere but the airport returning from abroad, and they surely have their reasons. But lots of people don't stay at the airport and they have never missed their flights.