Given you have travelled before you will have some idea of how to access the toilets etc and how you manage that aspect. How old is she / how big is she as that will be a factor in how comfortable you all are in Y.
I think seat selection would be worth paying for (if not covered by status etc) to try and optimise your seating.
A 4 in a row at the middle bulkhead (non-exit row) or as suggested 2-2 in a row - if you can get upstairs on the 380 the layout is 2-4-2.
I'd also suggest a call to QF (assuming it's QF, or failing that, whomever the airline is), if the online tool won't assist with decent seat selection and see if they can "override" the selections. Otherwise if it's 3-1 seating you end up with, then the three of you can rotate turns in the "1" seat to maximise your rest time.
Failing all of this, a casual conversation with the seat "mates" around you early in the flight just letting them know the jiggling is involuntary, and the vast majority of people are understanding. If not, speak to the FA onboard and see if you or more likely the other pax can be repositioned elsewhere in the cabin to make everyone's trip easier (if there are available empty seats).
Good on you for worrying about others but i'd personally say you've got enough on your plate and try to do what works for you. If other pax don't like it, and you've taken reasonable steps to make the trip easier for everyone else, there's not much you can do.
