I'm not a rope dropper either these days - lol! I would be though if I wanted to ride a lot of popular stuff without a long wait cause that's definitely the way to get it done. As long as your sister knows that ride options are more limited with longer waits then that's all you can do!
I've also not stayed at the Poly (have done CR before the
scooter and GF with the scooter). I have never had a problem with the monorail and the CM's have been great. I think off times help with the busses but you also run into the issue of fewer busses - yet I've never had a problem that was due to being in a scooter - as in I've rarely had so many scooters I couldn't take the first bus. I also don't think AK will be that bad leaving in the evening since departure from there seems to be a bit more staggered. The tough times are after the MK fireworks and Epcot Illuminations. I'm not sure how evening shows at DHS affect the crowds.
I also need both knees replaced so I know how that is. If your dad can walk some - I know I have times when my knees are terrible and other times it's ok - then he can feel free to park the scooter and walk a bit. I often park to go into stores to shop if I can. I enjoy shopping at WDW and maybe your dad doesn't so much...but if he does then the best time to shop is earlier in the day when the stores are less crowded if he needs to stay in the scooter. Towards closing time the shops get very busy and taking a scooter through them is tough. I also recommend that he take the scooter through any ride ques that he can - even if you have a fast pass. You know that some ques are quite long (Soarin, stuff in Pandora, etc). I'd rather be able to use my standing and walking time in shops and restaurants than ride ques. However I generally do park the scooter and walk through the que at Buzz IF I have a fast pass because the que is short with FP and it requires transfer to a WC. Haunted Mansion is a tough one - they ask you to park the scooter and transfer to a WC too. I usually walk through if I have a FP (honestly I just don't ride it if the line looks long lol). That's a longer walk and stand than I'd like sometimes. Other rides where I may walk is Splash and Big Thunder Mountain but those are tough and with stairs - and only with a FP and if my knees are feeling great. They both also have an entrance where you can stay in the scooter and avoid stairs.
As I type this I realize that I tend to ride stuff that I can get a FP for and stay in the scooter for the que (hello Little Mermaid -- haha!). Also there have been some threads here about getting in and out of rides for those who have knee issues. If your dad can't bend his knee too much then some rides will be tough anyway... like 7DMT perhaps (I never have issues), Splash (this one kills me - I rarely ride it now), IASW (the seats are so low and cramped), Astro Orbiters, and probably others. I can do FoP though but it's close cause of the knee bend. I'm not sure if you know what he plans to ride but you may want to think about it if you think he'd ride a lot.