On the "straddle" period, my understanding is that there are 2 things to keep in mind. When you do a formal "tour" with a guide (which can be virtual), the offers you receive are good for 7 days - I haven't confirmed this, but I don't think that changes if you do a tour in the 6 days leading up to incentives changing. The other thing to keep in mind is the 10-day cancellation period. You tell your guide verbally what you want to purchase - then they send you documents to e-sign. You have 10 days from signing those documents to cancel everything without penalty (and they will not charge your credit card until you actually sign). They will send you the documents to e-sign pretty quickly, but you can wait a couple of days to sign too, which can give you a couple extra days.
So, there are a number of ways to work it if you want to straddle an incentive period. I happen to to be traveling to WDW the week of 7/12-7/19, so I'm planning to do a formal in-person tour with my guide and get an offer that's good for 7 days (again, haven't personally confirmed they'll do this immediately before an incentive change date, but hopefully they will). Personally, I'm hoping incentives are better, in which case I will probably wait until September to move forward to save on a couple more months of dues. Of course, if you tell you're guide that's what you're going to do, they're likely to tell you the incentives can be pulled at any time, which is true, but probably pretty unlikely absent some really crazy fire sale type situations - hard to see that happening with Poly or RIV.