The more that I read your description, the more that I believe that this is indeed a problem with the switch. When you drag to select a passage, if the button switch is intermittent, then it will indeed select random selections of the passage, and will randomly reposition text if you are working in an app that has drag-n-drop text editing enabled. Your symptoms sound completely consistent with a noisy button.

On my mice I fully desoldered the switches, disassembled the switches, cleaned the contacts, and reassembled and resoldered the switches.

Here is a youtube video where the guy is doing it without the desoldering step, on what I believe to be the same model that you said your mom had:

https://www.youtube.com/watch?v=82z-OpHdBgE

Of course, the other, easier option: Get a new mouse.