Mmmmmm..... no.
DEC will depreciate vs. SPS as the AD value of DEC becomes less and less, as DEC depreciates vs. SPS- shares in the LP will actually appreciate as the SPS:DEC ratio automatically adjusts- multiplying your AD points over time.
Absolutely! it's a snapshot- and yet, Staking is a 4 week commitment. If someone stakes their SPS, today, they can't move it into the pool next week when the ratio changes so you definitely should have considered at least a week of compounding in your analysis.