Whilst having no experience or neither Kindles nor macular degeneration, the following occurs to me.
If your mother can cope with the large print books from the library then the Kindle may be of use for a while. Worth a test I'd suspect.
Also I would have thought your mother could learn and memorise the button functions, and where they were. Although I'd suspect with very limited sight it may be difficult to confirm the screen is at the stage you think it is. Perhaps try to get hold of one to look over, and see how important feedback from the screen is ?