That's why it would get screwed...
Well, it would be retconned, but at this point I think an on-screen retconning of the Infinity War references last year and an acknowledgement of the Snap n' Blip is probably the best we could hope for.
I've yet to be right, BUT I do like Slyonic's suggestion of universe hopping. Maybe the final season will see the cast hopping between alternate timelines? Per one interpretation of Infinity War's logic, they can't change the past, so there could be several in play:
Timeline 1: Original Quake timeline (technically the original MCU timeline), where the world was destroyed
Timeline 2: "Main" MCU timeline, where the Quake future was averted in 2018, but a duplicate of Coulson was sent at least 100 years into the past.
Timeline 3: Sarge timeline as seen from about AoS 5x12 forward.
Per this scenario the Snap n' Blip only occurred in Timeline 2. (Possibly something in the past kept Thanos from getting the Infinity Stones after his attack on New York in Timeline 3.) It's crazy convoluted, but it would allow them to show Fitz and Simmons living in the post Quake apocalypse of Timeline 1 again, answer questions about how the Blip impacted the team from Timeline 2, and allow Coulson a happy ending resurrection of sorts for the series finale in Timeline 3 if he regains some of his pre-Sarge memories, without leaving any version of him alive as a loose thread in the "main" MCU.
TC