The only restriction loading a file is that it has no more than 100 limbs and that you do not have more then 20 objects in the scene at once.
Saving and loading animations uses limb names to re apply animations to new objects. If limb names are the same then the load and save will fail. Also limb names should not have any spaces or the .3mp format will lose all other words in the label.
Mouse Tutorial is for creating video tutorials and showing mouse usage to help explain what is happening.
All media created with Model Prop should be saved as a native model prop file for easy reload and edit purposes. Model Props native format is .3mp and is text based for simplicity. Loading a .3XR is limited but can load some of the older files created with Model Prop before it was heavily upgraded.
Importing an object will load a limbed or no limbed object into the editor. The object will most likely fall apart until it is saved once and reloaded as a .3mp