Complex sentence: The interface had to contain lists of objects, characters, and other resources, and options to manipulate those resources, and be mostly accessible using only a mouse as input, reserving the keyboard for less common and more advanced commands.
Split into simpler sentences: The interface had to contain lists of objects, characters, and other resources, and options to manipulate those resources. It had to be mostly accessible using a mouse as input, reserving the keyboard for less common and more advanced commands.