So there is a lot here..
Lets start off with fixing the terminology. You get unlimited photopass downloads with a 365 day photo expiration with an annual pass. It is not memory maker but it works almost the same but a longer expiration date... Memory maker is a package that anyone can buy to get unlimited photopass downloads for 30 days.
Whoever has the annual pass and will be doing the downloading, they need to be able to see all the friends and family in the list in order to see everyones photos. So linking to a friend but not linking to the friends child means you may and probably won't see the childs. You actually have to do it in 2 steps now where first you link the person who has the mde account, then you can find and link all their managed friends and family to also link to them. It does require them to accept the invitation in order to complete the link and see the photos. This is probably what you needed to do stopping by a photo store so again if you notice any photos missing, this is the quickest way to solve that and I recommend you do that again if there is a problem.
As far as the magicband, first the person needs to be on the reservation to get a free magicband. You can add them after you arrive, which is a great way to add people to a room without the same ticket or meal plan. So if this is your plan to add her, then yes she can get a free grey band from, the front desk. you should be able to pick that up for her, when you go to add her to the room and shouldn't require her to be present.
And last a magicband is not required to use photopass. The friend can just use her plastic ticket for photopass and that will work just fine. Since she'll be joining you on the trip, the other people with their magicbands will take care of the ride photos getting automatically linked.
Hopefully that helps you out.. It's really simple, everyone needs to be linked, photo pass store/kiosk can help you fix anyone you had issues or forgot about, the friend can use the card or magicband if you add them to the reservation for a free one or buys one and sets it up with her account.