The way to do the desired booking and face no issue of losing part of your reservation at 7 months out: (a) make two separate 11-month out reservations at home resort, one for nights that you will keep for home resort and the other for nights you may want to switch to another resort; (b) online at 7 months out from the reservation you want to change, try to modify it to another resort; that reservation for your home resort will be kept if you cannot make the modification.
Thereafter, if you are unable to make the change and are resigned to keep the entire time in your home resort, contact MS and have it merge the two home resort reservations.