Conclusion:
Weapon Elemental Gauge Units or WU, can be used to describe the strength of Weapon infusions. Keqing infuses her weapon once on double casting E and Diluc infuses his weapon every 0.5 seconds if it isn't imbued with Pyro. The duration of these gauges differs between characters. Reactions on weapons follow the known reaction modifier rules from other types of elemental application, and similar to Self-auras, can oscillate directly between elements without shifting to physical.