Love the idea, and honestly think it's the best way to go with Quickness Herald. Fully agree with all the points you made as well. 6-pip upkeep cost overall just feels terrible, especially on Glint where now you have to micromanage your active facets just to maintain quickness. And I'm sure I'm not the only one who would rather just run Shiro/Jalis, as boring as that is.
I'm not sure offhand about the numbers, but I feel like 1s duration is fine. Keep in mind too that True Nature will leave us dead in the water for at least ~16s. Plus, being able to temporarily overcap on Quickness would allow us more wiggle room for utilities on other legends.
Also, I don't think what you proposed is too strong. For comparison, both Harbinger and Deadeye can both currently provide permanent Quickness uptime with little to no boon duration investment. Not saying Herald needs the same treatment, but the tradeoff in how you gear should be qDPS vs. qHeal/Support.
Hard agree. All the core Revenant skills need some cost reduction/reworks to be honest. Rev has too many dead buttons, and Shiro is by far the worst offender of this.