Barrier's default natural decay is a percentage-based value, roughly 9% of max barrier per second, i.e. all barriers completely decay after 11 seconds (less with damage) regardless of level or strength, therefore it does not change throughout the game. Knight-Protector's 35% decay reduces this to about 5.85% per second (9 x (1 - 0.35) ≈ 5.85), i.e. barrier now completely decays after 17 seconds, a 6-second increase
Simply having a Knight Enchanter in the party with this ability learned will reduce all mages' natural barrier decay by 35%, not just the Knight Enchanter's barrier as the description might imply
Having both Vivienne and a Knight-Enchanter Inquisitor with this ability at the same time will not provide stacking effects.