It is passive. Neither the player of PC is choosing to do anything, so there is no disassociation. The character has the spider sense or something.
This is exactly the kind of post facto explanation that hallmarks the claim of dissociation! Why is the character not surprised? Mechanically, we know it's because the player chose Alertness as a feat. The player made a choice. Fictionally, we have no idea, it's spider sense or something. It's unexplained, or forced to be explained ad hoc in the moment if it's not just (as usual) ignored as okay.
I don't know. Seems indeed somewhat disassociated. or weakly associated at best. I would describe it as the character consciously drawing on some reserve vigour. And why only fighters can do it? Presumably for the same reason Usain Bolt can run really fast and I can't. Some people simply can do stuff other people cant, most likely due being gifted and practicing really hard.
Resting. But yes, the use of hit die is disassociated. It would be more associated if the healing was passive. I use healing kit dependency, so there at least use of hit die is associated to actual first aid, though decision of how many hit die are used is still disassociated.
I describe any hit point loss being at least somewhat hurt. How badly you're hurt is associated to the proportion of hit points you've lost. I liked 4e healing surges for this reason, as they allowed healing to be proportional too, and I wish 5e had kept that.
Okay, you've made a change, but one that seems hard to do. What the descriptive effect of losing 1 hitpoint if you have 10 vs if you have 100? It can't be just wounds, because the same blow can do the same damage against both opponents, but the descriptions has to change. This isn't about following PC wishes but about having to rationalize a mechanic.
No. This is not disassociated. Like Usain Bolt can run a lot faster than me, some people can learn to punch much more rapidly than others.
Only fighters? Rogues can't be quick? What explains the speed of attack, here -- is it even speed? I mean, if I have a 20 STR 20 DEX rogue, why do I only get one attack a round but the 10 STR 10 DEX fighter gets multiples? It isn't strength. It isn't speed. It isn't skill (we both have the same proficiency bonus to attack). It's just because one is a fighter and the other a rogue? Okay, sure, fighters might be just that much more preternaturally skilled, but now we have to explain sneak attack in the same paradigm -- rogues are the only ones capable of such precision and overwhelming blows?
Yeah, it's entirely dissociated from explanation.
Why doesn't everyone do brain surgery? Because it is actually hard. This is not disassociated.
Sure, but we aren't talking about that, are we? Every character in 5e is just as proficient at a given level of landing a blow with a weapon -- the proficiency bonus (that skill and training bit, per description) is the same. A Wizard is as good at swinging a dagger as a Fighter at the same level and with the same stats. Clearly both the Wizard and Fighter can try to stick someone in the tender bits from hiding just as well as the rogue can, right? Same same same. But, the Wizard, with the same prof bonus, gets 1 attack at d4, the Fighter gets multiple attacks at d4, and the rogue gets one attack at d4 plus a bunch of d6s. Does the rogue know something about fighting that the Fighter does not? Kick the Wizard to the curb, maybe (oh wait, he has the criminal background, or the solider background, so grew up with that training!).
Yeah, the entire structure of class abilities is dissociated. It's not effected based on what the characters choose to do, but what the player selects as the arbitrary bin of powers that the character has. A Fighter in a fight is choosing to go all out to survive and win, just like the Rogue, but the effects are not based on these choices but rather some arbitrary player-side choice and design choice. Totally dissociated. It is, however, what we're used to, so it doesn't get noticed and, if it does, usually gets excused away.