I am surprised that the BW knows now that they won’t have availability in October - sounds to me like they overbooked during the very popular Food and Wine plus now GE opening. That said, kudos to OP for having such a great attitude!
I think you’ll enjoy the GF. I suggest planning for your sister’s family to hang out at the GF with you on their MK nights (taking the boat from the front of the MK to the GF on a warm night is a delight, watching the MK fireworks from the terrace by the quick service restaurant Gasparillas or the GF dock is pleasant as well, sitting in the GF lobby listening to the band or the piano player is very pleasant as well.) On Epcot & DHS nights you can visit them at the Boardwalk.
Getting back to GF after your visit at the BW, check the schedule for the MK, if they are open late even for an after hours Halloween party the buses from the resorts to the MK will be running, if so when it’s time to leave just walk down to the BW bus stop, grab a MK bus and once at the MK boat or monorail to the GF, or as mentioned walk or boat over to DHS if it’s open late and bus back to the GF.
And as mentioned, if you just don’t want to deal with security going via the MK or all of those transfers Uber/Lyft are reasonably priced options.