We will be there the last Saturday in October. My kids will be 5 and 7, so I don't know if the single-rider line will work. Maybe for things they don't want to ride, like Dueling Dragons. What happens at Ollivander's? Is it worth the wait? Is it expensive?
After HP, we mostly want to do Juarssic Park, all of Seuss, and Rispsaw falls. Maybe Spiderman, but not Hulk. I have two girls and none of us is really into comic books, so I would be okay skipping that whole section.
If we do stay off-site, any recommendations as to the best hotel as far as convenience and amenities? Also, I know nothing about City Walk. With two younger kids, should that be something we are thinking about for Saturday night?
My wife and I swap out for single-rider so that one of us is always with DD6. That is only necessary on the big thrill rides, since DD6 is either not tall enough or daring enough.
Ollivander's is a recreation of the scene in the first HP movie where Harry gets his wand. It is cute and mildly entertaining at best. Having done it twice, I would say my family would wait maybe 30-35 minutes at the outside. If you've never done it and have time to kill, then 45-50 minutes tops. In late October, crowds should be fairly light and I would guess you would have a good chance of a shortish wait in the last hour of the day. They cut that line off before park close, however, so be careful not to wait too long to get in line if it's a priority for you. Ollivander's does not cost extra, in fact you can't buy anything in there either. After the show, you are ushered out into a store where you can buy all kinds of wands and HP swag.
Jurassic Park is fun, my kid loves the play area in there (Camp Jurassic). Pteranadon Flyers is also a fun ride, but is slow-loading and you must have a child below a certain height to ride it. Adults by themselves are not allowed to ride. I also like JP River Adventure, but one or both of your kids might find it a bit scary towards the end.
Ripsaw Falls is a blast, and DD6 loves everything in Seussland. She did Spiderman for the first time, but spent much of the ride with her head in her lap. IMO, it is not as scary as JPRA.
CityWalk to me is a louder, more adult version of Downtown Disney. We are not clubbers, so we skip all that and just hit Bubba Gump Shrimp Company for dinner and head out. If parking at Universal, then you have to walk to CW to get to either of the Universal parks.
Sorry I can't help you on nearby hotels. This last trip we stayed near Downtown Disney and drove the 15 minutes or so to Uni each day.