To prevent map2 from gaining another "imageCollection" tileset, it would need to contain exactly the same images as the one in map1. A better way to deal with this situation is to store the image collection tileset in its own file (a .tsx file, generally) and add that collection to both maps. The name "imageCollection" is the name of the tileset that you chose when you created it. It can be changed in the Tileset Properties (menu Tileset > Tileset Properties..., available in tileset editor, which you can open with a little icon below the tileset view).
Regarding the crashing when deleting more than 3 or 4 images, please open an issue about this on GitHub and include the steps to reproduce the crash. Thanks!