To begin with, there is no 2/29/2017. Feb ends on the 28th this year and to reserve a room beginning Sep 29, you need to wait until March 1. Yours is a difficult situation. Typically the way to book more than 7 nights at 7 months out is to book 7 nights exactly 7 months from date of arrival and then add an extra day by calling daily, e.g., for 10 nights beginning Sep 22, you would, for example, reserve Sep 22-29 on Feb 22, and then add the night of Sep 29 on Feb 23, the night of Sep 30 on Feb 24, and the night of Oct 1 on Feb 25.
But you have three major problems following that process. One is that you want two rooms so that your group can stay at the same resort and, in your case, you even want them to start on different nights. That means you could face the issue of switching one room and then possibly not being able to switch the other. The second issue is that your existing 15 night reservation that you currently have for Kidani is a single reservation and thus you have to cancel it entirely to be able to switch and use any points in it for the Poly reservation, and in doing so you face the risk of switching to Poly for 7 nights but then possibly being unable to add all of the remaining 8 nights (and then also not being able to go back and reserve Kidani for all the nights if you cannot book Poly for all the nights). The third issue you have is that because the time of year you want includes late September and the first week of October, you have a real risk of both the first and second issues actually happening, i.e., days in late Sep and first week of Oct may fill at Poly before you can get those days.
Right now you are on the cusp. DVC's low to moderate demand season at WDW, which started the day after marathon weekend in Jan, will end within the next week, and the high to extremely high demand season will begin (and run until marathon weekend in Jan). Poly may last for you to be able to get it and it may not. Though all dates may be shown as open right now at Poly, what likely exists now is that there are many more rooms open for Feb 22 to about 25 than there are for Feb 25 to Oct 7. It was open at 7 months out in 2016 for all the dates you want until February 27, 2016, at which time the Poly studios filled for Sep 27 and 28 (and also filled on for Sep 24 and 25). Those studios then filled for Sep 29 and 30, 2016 within moments after 8 a.m. on Feb 29 (i.e., only those with the fastest computers comunicating with Disney's got those dates), and that kind of instant booking at 8 a.m. continued into the first week of Oct until the week was gone. Poly also has the issue that it continues to sell and as it gets closer to sell-out, availability is going to get tighter and thus this year could even be more diifficult than last year for the high demand season.
As I see it, you have the following options, and I am assuming here that it is most important to have all your group together at the same resort:
1. Forget about Poly and keep what you have, the safest course. If you want to do two rooms at Poly reserving 7 months out, one for 15 days and one for 7 days with a different start date, plan to do a vacation that begins and ends sometime between the Monday after marathoon weekend in Jan and the third week of September when it is generally easy to get Poly at 7 months out.
2. Wait until March 1 to attempt the switch. Significant risk that won't work because at least some of the days you want at Poly will be gone, but at least then you will not have lost any of the Kidani reservation. Note, however, if you actually see that all your dates at Poly are available at 8 a.m. on March 1, 2017, you still have a real risk of not getting Poly for both reservations, i.e., you might switch yours to Poly but then find you are closed out from switching your SIL's reservation.
3. On February 22, if Poly is open for all dates you want, cancel both your existing reservations and reserve two rooms at Poly for 7 nights. Then for one room, call MS daily and add a night unti you have your 15 nights. For the second one, call daily and drop a night at the beginning of the reservation and add a night to the end and continue to do that daily until you have Sep 29-Oct 5. Doing that will not assure you will get the rooms because someone who owns at Poly could fill dates in Oct before you can get them. However, it will assure that any others trying to book Poly at 7 months out for any dates from September 22 and after cannot prevent you from getting all your dates -- e.g., once you reserve a room for 7 nights, Sep 22-29, no one else using the 7 month window can get the room you have on Feb 23 for Sep 23-30, or Feb 24, for Sep 24 to Oct 1, etc., because your room is full for their start date. Again if you follow that process, someone with the 11 month window could stop you from getting all the nights you want, but no one using the 7 month window could stop you, and it is likely the persons who would prevent you from making the switch on March 1, if you tried method 2 above, will be those trying to reserve at 7 months out. You are still acting with a risk of not getting all your nights but if you have Kidani savanna view now, you would still have a decent chance of getting it back if you cannot fully book Poly by March 1.
