Monohull dinghy with amas above waterline just for stability

The main problem is that you won't want the amas there once you've learned how to handle the boat well and how to right it after a capsize, so it's just extra expense, clutter and hassle. If you have a small ama there it can still be pushed underwater and will make the boat more unstable at those times rather than less, and there's a significant chance of the boat ending up upside down and being impossible to right. A simpler solution would be a masthead float or semi-inflatable sail to stop it inverting, and a pole with a relatively light lead weight on the end that can be swung or extended out in the opposite direction to the mast to right the boat. I'd look at building something like that into the centreboard such that the lead weight is a bulb kept at the bottom of that and can be extended down on a telescopic pole, while it could also be removed and replaced by a standard centreboard for racing. While not extended it would still provide lots of extra stability that would make sailing more comfortable for beginners.

 

A boat such as you imagine will have numerous negative unintended consequences for dealing with mainly 'what if' situations.
Simplicity is an asset in dinghies. If you want to trailer the boat it will have to have foldable or demountable amas. The sailing rig will have to be made heavier.
Boarding and disembarking at the dock will be a big bother. The boat will have handling issues as it flops on and off the amas. Weight placed away from the center of the boat affects motion, handling and stress on the rig..
With all due respect, learn how to sail a dinghy. You do not have the experience to improve them.

As far as safety, you want good flotation and the center/daggerboard trunk top opening to be above water when the boat is righted.
The following video is of a sister ship of mine and pretty much the gold standard for non self-righting sailing dinghies.
To save time start at 6:06.

 

OK, thank You for the explanation and the interesting ideas, now it is much more clear to me.

 

My first sail boat had a ballasted keel. Going to an unballasted dinghy sure took some getting used to!