Can look at Hotel MYSTAYS Maihama, can walk to DisneySea. That way if your mum is tired, not too far to go back to rest.
https://www.mystays.com/en/hotel/urayasu/hotel-mystays-maihama/
good reviews on Tripadvisor plus got massage chair lol they have shuttles but only in morning and PM but if take taxi also wont be to ex.
last 2 weeks of April looks to be low season (based on Apr 2017) so I think you will be ok in terms of crowds. I won't really do Disney on my first day but I would rather visit on a weekday than a weekend.
I still think Disneysea is nicer...really more for adults. Disneyland is more for kids.