How to let my mum switch rooms with me (Long but please read)?

I think you should see what it is that you really like in your mom's room that makes it more relaxing for you to sleep easily in and then try to incorporate those colors or themes in a different way into your room. Maybe you could also rearrange your room differently to change the floor space. I do see that you like the location of her room better, however, you should not consider the larger room - she is the parent and she deserves the best space.