I'd definitely plan on 3 days for Disneyland--remember, there's all of Cali Adventure, too, and they just remodeled about 18 months ago (2 years in July or August). Bugsland (at DCA) is GREAT for 3 and 4yos, and you can spend hours there! Carsland also has Mater's Junkyard Jamboree, which is pretty fun for smaller kids. And then if your DD knows Ariel, she has a whole ride, too... And the whole Redwoods Challenge area--great for getting pent up energy out!
HoJo will let you store your luggage, I believe, without much of an issue. I know that's fairly standard for most of the hotels in the area. I've heard that some of the rooms have been remodeled, and others haven't, and that the ones that aren't updated are pretty dated. I'm not sure about seeing fireworks from there, but I think it's completely possible. I know we were able to see them from Katella and Harbor without a problem. Granted, that's the other way, but it makes me think you should be able to see them at the HoJo without a problem.... It could definitely be splash-pad weather.
As far as your mom's arthritis, would she consider a scooter? It could make this trip a LOT more comfortable!
--Mimi
Mom to Lala (2004), Bonus Mom to Big Sis 1 (1991) and Big Sis 2 (1992)
Grammy to Big Kindy Kid (2011), Big Pre-K Kid (2012),
Grandbaby Appendage (2014), and New Baby Grandboy (summer 2017)