I was an organ tech for ten years and here is what I would do:

Turn the keyboard upside down and carefully remove the screws holding the bottom on.

Usually there is a rail under the keys that holds them on. Remove the screws attaching the rail.

Carefully remove the section of keys that are sticking.

Using a pipe cleaner and "Denatured Alcohol" clean off the gunk that is causing the sticking.

If you can get at the "D" button contact spray a little WD40 on the contact. Under no circunstances do you ever use "contact cleaner" or TV tuner cleaner. These types of cleaners contain silicon and it will ruin the little carbon impregnated contact. You may have to see if the plastic "D" button cover will pop off and get at it that way.

Reassemble it. As long as you keep yourself free of static build up by first touching something metal you really shouldn't hurt anything in there. Most everything electrical is on one circuit board and there is no need to mess with that. The key sticking problem is mechanical and at most the "D" button has a worn contact but it probably is just crude.

Tom