Yep there was a time when Reed Richards and Ben Grimm were WWII vets and Charles Xavier fought in Korea.
Absolutely. And Marvel maintains they’ve never fully rebooted, they’ve just retconned timelines and elements. Though Universal, and Multiversal transformations of reality have occurred, and just as Prodigy co-show runner and EP Aaron Waltke has invoked the Temporal War in Trek, they can tidily explain their retcons.
And regarding your examples, most recently under Mark Waid (2019’s History of the Marvel Universe) and Kurt Busiek (the current ongoing sequel to his & Alex Ross’s Marvels, The Marvels), it’s put their military service, or in Reed’s case, now having worked as a consultant rather than a service member, as associated with the fictional South Asian Siancong/Sin-cong War. This included not only the FF’s Ben & Reed, but former Iron Man, James “Rhodey” Rhodes/War Machine, and Frank Castle/The Punisher as well.