Bottom line up front: I see a lot of armchair speculation and overgeneralization, but little evidence of extended critical listening and careful comparisons.
Underspecified. Do you mean FR at a static point in time or FR in the real world, with drivers that all have distinct resonant properties and decay timelines?
That is tautological. Yep. The headphones “acoustic system” is responsible for the technicalities of the headphones.
Impossible. Different drivers have different sizes, weights, materials, compositions, etc. As above, a point-in-time FR doesn’t equate to identical real-world musical time sequences. See the ASR crowd…do they seriously try to make their Porta Pros sound like Utopias?!?
Now, this may not apply to brands that seem to sell maybe 3 families of drivers and tune them very slightly differently for different users and price points (e.g., Focal, Hifiman, Sennheiser). In those cases one may indeed come very close to duplicating the performance of another product. And if a maker consciously tries to clone another product then “different drivers” could be tuned to be functionally the same.
Yes. That’s why EQ has value.
NO NO NO NO. BS BS BS BS. NON SEQUITUR. This is ASR-like armchair speculation. Listen with your ears. First, FR in #3 may look “fine” with a static measurement tool, but “exact” matching is impossible over a time sequence. Second, dynamic drivers routinely take on the resonant character of the materials of their composition. Paper cones sound “papery” – soft, dull, rolled-off treble, plastic cones sound “plasticky” - hollow, boomy, low-mid emphasis, and metal cones sound “metallic” – bright, precise, reflective. Planar and electrostatic drivers tend to sound diffuse and dithered versus the single-point movement of a dynamic cone.
I swap you my Koss Porta Pro for your Focal Utopia. Even trade. 
So, you just admitted that #5 is BS.
Again, you admit #5 is BS and replicate my comments about driver construction and composition.
It is rather easy to identify headphones with the right testing method. I, and many people, tend to fail rapid ABX testing between setups. But, listening to a 10 to 30 second clips one after another does not allow psychoacoustic habituation to occur and does not resemble how music is actually consumed. As such, I created a marathon playlist to address the only property that cannot be ignored, denied, or avoided: fatigue.
Some music sources are recorded poorly or have baked-in noise so they make my ears ring or give me a headache. The key for testing is that the impact changes with the setup (DAC, amp, headphones), and some headphones pass my marathon test while others fail in seconds, or after 30 minutes, or after an hour, or after 2.75 hours. Each set of drivers has little tiny unique quirks and responds differently to some notes and transitions – these key points can be learned whereby any changes with a new set of headphones stands out.
This method cannot be blind, as pain and ringing ears are inescapable. The time point and type of pain becomes a fingerprint for the driver. If you use this playlist or a similar one, test with a clean DAC and a solid state amp. Some differences will be obscured by colored setups, tube amps, etc.
That’s uncontroversial. EQ can shift the coarse properties of headphones in any given direction. EQ can’t make a woody avocado from Peru taste like a sublime creamy avocado from Mexico (true story: check the labels). EQ can’t make Budweiser taste like Budvar. My offer of giving you my Porta Pro in exchange for a Utopia stands.
And so, what was the point of this armchair exercise? Coarse tweaks work well. Yes. Nuanced tweaks and “free upgrades” to flagship products fail. Yes.
As in my prior response to you, this generally means tighter tolerances and better quality control. There is no way to make a cheap mass-produced driver be as consistent as one built with care from the ground up, and built with higher quality materials. The basic HD 600 is an excellent product. I am routinely shocked at how good it sounds when I go back after a break. Still, it doesn’t have the range, blackness, foreground/background distinction of my Clear. My Clear doesn’t have the range or nuances of my HD 800 S. While I can’t handle the Utopia’s treble spikes, it greatly outclasses other headphones in 3D space and localization. No other Focal product can touch it or be EQed to come close.
There’s all sorts of luxury branding and image-driven price mark-ups with audio, watches, jewelry, Lexus vs. Toyota cars, and more. I downright hate some flagship headphones (e.g., everything with closed cup), and don’t much like others (e.g., Susvara). The minimum price depends on your interest and ability to hear nuances. For many people this could be the HD 600 or HD 650, but the HD 5 series probably won’t be good enough. Still, I’m quite happy for upgrading beyond this tier. It’s a hobby, and the more expensive headphones perform substantially better with my marathon tests.
See above. Sometimes yes, sometimes NO NO NO NO NO. Overall I think you are slipping into the ASR “equal measurements mean its just as good” error. No, listen with your ears. I own two amps that fall near the ends of ASR’s amp measurement ranking list (the last time I checked). Thse include the “terrible” Bottlehead Crack and the “excellent” THX AAA 789. I still use the fuzzy Crack but boxed up the 789 (and would sell it if there was much of a market for them). The 789 can sound like listening to sandpaper on broken glass while sliding across ice. But its measurement numbers are great.
Personal testing. Nothing else matters. Your hearing and that pleasant zing you feel from “cool looking” equipment is all that matters. About 90% of the technicalities can be reached with the HD 600. If you are value oriented, stop there.
You can downgrade sound easily, but you cannot upgrade sound quality a great degree with EQ. That’s wishful thinking. About 1/3rd of the tracks on my playlist are heavily distorted and degraded – this is to show how different headphones perform with less-than-ideal sources. Some tracks can be indistinguishable with cheap vs. expensive headphones, or they result in very different whines, whistles, missing bass, or artifacts.
This is perception science and it’s a well researched domain. I think you are again getting at sustained FR over time, not static FR charts. The A90 vs. RebelAmp comparison follows from materials and construction – the RebelAmp is hugely overbuilt (I own one) and can deliver plenty of power/current without breaking a sweat. A weak amp, say from a tablet or phone, typically struggles to deliver current to demanding headphones and sounds terrible. See the many discussions of Sennheiser “scaling” and “synergy.”