I think you've answered your question. 1 GCD is 2.5 seconds, you have your window there.
You are also not the only one on this boat. I personally hard cast RS + Potion so I lose time and it feels weird but the damage is here.
A stat does not magically makes a certain fight better. I still fail to understand why you think A1 doesn't reflect the reality. Everybody gets more DPS on 2 targets and stat weights does not matter. You multidot on 2 targets as a Bard like a Summoner would. Like a BLM would Flare at the end of his rotation or a Dragoon would Chaos Trust both targets.
Well, the Bard on the video is form your Free Company, maybe you should ask him.
If you fail to perform like other Bards, the producer did mention Bard was working as intended and to deal with it.
I'm sorry about the question earlier but you still didn't answer it. I see no math that prove Bard is broken right now. There is no PPS (Potency per Second) You do not show with maths why Bard feels clunky. From what you told me, your skills take 1.5 seconds but triggers a 2.5 seconds GCD. It's enough for 1 OGCD. If multiple CDs happen and you have low latency, you could probably inset 2 OGCD losing minimal time. I personally don't see the problem. I'd love to have my spells be 1.5 secs and trigger a 2.5 GCD, unfortunately the BLM spam spell is a 3 seconds cast time. If I can deal with it along with my OGCD, I don't think you have the right to complain. Even if you have 11 OGCD.
If I were you, I'd try to math out the lost on OGCD before doing any claim :\ I would also question the Bard in my FC that is doing 1239 DPS in A1 what he is doing.