Attempt to modify the standard RMG.

You've done a lot of work, but I have to disappoint you. What you found in types.xml is simply the definition of editor fields. In other words, if you delete the line with NewShopBuilding, the tab allowing you to change that line will simply disappear from the editor.
Essentially, all properties specified under such headings are only needed so that the map editor understands which file/element it should contain in this field and which element it should create when clicking New.

I played around with them once; I managed to add new fields (for example, 3X3SizeCreatureTexture ), but I wasn't able to bind them to the functionality (as in the code in your second example).