What possible way can check to tell if my overnight battery drain is due to the diode sticking? Is there a test or a tip you could offer? The battery is new and the alternator tested ok.

My tip is for all makes and models of vehicles
It is a common fault these days are more and more extra fittings are loaded to the cars and when proper precautions are not taken it will end up to fault the system.
Hence in case of a battery drain overnight try these checks.

You must check the drain current on your battery after the engine is switched off. The best way to check this out is to connect an ammeter in series to the battery and check. There can be a permissible limit of few milli amps but a large drain can in about 12 hours ( overnight) fully drain the battery no matter how much you have charged it or the system charged it up.
Now disconnect one by one the circuits to centre on the faulty circuit and then the faulty device. This is easy as the fuse can be pulled out one by one till the ammeter reading drops down.
Now it is always possible that directly connected circuits like Clocks, audio/video systems, remote control units , brake lamps park lamp circuits can be possible problem areas.
After the device is identified and battery must be charged up fully and tested overnight.