I don’t know that I understand what you’re asking about the cool whip or milk. For me, if I’m low before bed, I have a tendency to drop slowly through the night, so I most certainly need to treat the low - with whatever seems like the best choice. Right now, milk is a great option for me because I’m trying to get more calcium in my diet.
As far as how fast one can drop, that varies not only from diabetic to diabetic but situation to situation. There is NO answer for you. If I have insulin on board, I could see that drop in 15 minutes or I could hold at 70 all evening. If it’s two hours after eating, depending on how early you took your insulin before the meal (I do mine 20 minutes early, usually), that insulin is in your system for at LEAST another hour after that reading of 70. (Fast-acting, depending on the insulin, can stay in your system anywhere from 3-8 hours, with the average being 4-6). Makes sense that the sugar would continue to drop. My endo expects me to be at 120 two hours after a meal and near 100 (my optimum target) after another hour.
(Lastly, and this one TICKS ME OFF, did you know that the first ingredient in SUGAR FREE cool whip is corn syrup???)