Can you complete old monthly rewards? For example, if we are in May and the May monthly rewards are released, can I still complete the April one? Do we have to be in the month we are completing?
Based off how it is right now, then yeah you probably will be able to do it later. The moments will stay, then the missions would still be there.
No these moments last all year. No rush at all. Last year I waited until October when the prices for collections went down to finally get Bellinger/Bell and Blackmon.