The details of how they get home are never covered except in vague generalities. They only tell you enough to know that they traveled for many more years, lost some friends, gained some new friends, developed, found and implemented new technologies, and eventually found a shortcut or two home. It's never mentioned when those new technologies were developed. Maybe they only had ablative armor for a couple years.
As for the Borg, they are only able to adapt to technology that they are able to assimilate knowledge about. They lack creativity to imagine new technologies and there are races advanced enough to prevent assimilation, unless the Borg assimilate some technology elsewhere to give them an advantage. If the Borg are never able to assimilate someone from Voyager that has knowledge of the technology or someone who understands another technology that would allow them to bypass those techs, they would not be able to adapt.