@Hannibal:
Not choosing and picking.
Any scenario where Observation Haki is used can be explained with the premise made.
1. Luffy stretched his arm backwards - Luffy doesn't exactly have to focus about his arms springing in front of him because Rubber is elastic, the potential would obviously catapult his arms forward naturally - so he doesn't even have to focus about attacking during this period.
2. While his arms are behind him, he has the time to focus on Mihawk and use Observation Haki to predict how he would attack and as a result, see how it was all going to end.
It's because Luffy was not committed to that attack (or attacking at the same time) that he was able to use Observation Haki. It was 2 distinct motives there: initially to attack > then focused on Observation > not both at same time.
If I'm picking and choosing, explain why Mihawk failed to hit Luffy every single time.
Explain why Sengoku couldn't tell Luffy was going to expand if he could use Observation Haki whilst attacking.
! http://i.imgur.com/0yYijGB.png
Explain why even before this chapter where he's apparently compromised by sugar, he failed to hit Luffy on multiple occasions whilst attacking?
! https://i.imgur.com/vZQQTof.png
Explain why Katakuri missed Sanji while he was attacking; coincidentally, Sanji who is not attacking is able to predict the shot.
! https://i.imgur.com/YPSUntE.png
All the times Observation Haki has ever been used were in scenarios where the user was just observing:
https://i.imgur.com/KVm6mla.png
Like here. Katakuri is not attacking. He first Observes Sanji, his prediction is accurate. As soon as he attacks, what happened to his accurate clairvoyance? Did he suddenly have an intake of sugar that Oda didn't show before he shot Sanji?
Or, where he shot Jigra, what was he doing? Just observing. Not attacking. It's after he observed that he shot Jigra.
https://i.imgur.com/4oKOMGN.png
Or look at Marigold or Sandersonia. It's clear they know how to use Observation Haki and this was before Luffy used Gear second in chapter 519 where he was getting overwhelmed. How come they can't accurately guarantee landing an attack on him? However, when they just Observe Luffy whilst not attacking, they're able to predict anything he does.
Look at any scene where Observation Haki is used. When the Observation Haki user is not attacking, they're omniscient, they know everything the observed will do. As soon as they attack, it's all down to luck. They can't guarantee anything. It implies it can't be used. It's more apparent when even the Fleet Admiral is shocked by Luffy expanding like a Balloon, something even Sataori was able to predict when he wasn't attacking, but observing.
! http://i.imgur.com/G2jApRG.png
If what I'm saying is wrong, it'd be inapplicable. However, evidence that suggests one can use Observation Haki whilst attacking simultaneously, I'm yet to see proof of that.
–- Update From New Post Merge ---
This is a discussion board no? If you think you know something that helps others understand better, you share it and scrutinise it to test how viable the explanation is. I'm just doing that. If I'm wrong, then at least we would have eliminated a perspective which would draw us closer to a better answer. However, if I'm wrong, then we should see Observation Haki users never miss attacks, and more so against people who aren't proficient or able to use Observation Haki.
Shura vs. Ganfall, why is it that Ganfall was fighting on par with Shura and even landed a solid blow?
Why is it that Mihawk failed to hit Luffy even once in Marineford despite trying to kill and cut him; only thing that saved Luffy was his unconscious use of Observation which means Mihawk's intent was the real deal.
Luffy only used Observation Haki twice against Mihawk in fact. When he wasn't using, Mihawk missed every attack, why?
! https://i.imgur.com/hq774hv.png
- Look at the panel again. He uses CoO after his arms are already shooting forward. Not while his arms are spinging back. It's very clear.
- He's fully commited to the attack. So committed, in fact, that he can't even cancel it. He has to change the direction of the attack to avoid getting cut. If he used it before his attack, why wouldn't he just not attack? Look at the series of events.
He shoots his arms forward, uses CoO, and changes the destination of his attack. He isn't using it as he swings his arms back. Which is what you're arguing. It clearly shows he's already in forward action. A punch is a punch. If you say he doesn't have to think of where his attack goes because he's elastic, then what exactly counts as fighting to you? Throwing a punch with or without being rubber is the same concept, only that being rubber allows the punch to stretch and go further. Once again, look at that panel. There aren't two ways to interpret it.
I'm not going to go through each one and explain it to you, because your argument is CoO can't be used simultaneously while attacking. Which Luffy clearly displays use of CoO while attacking lol. And while Haki in general is inconsistent, one example of someone using it during an attack destroys your whole premise. The Marineford Luffy example shows what you're saying is wrong.
Most of your examples of why people didn't use CoO, like your Sengoku example, is simple. They just didn't use it. Not that they couldn't.
You've seen proof, but it just doesn't fit your narrative and you likely will keep refusing to accept it.
However, you repeatedly deny concrete evidence, and haki as whole is too inconsistent to make up some absolute rule for, so continuing this debate is pointless