Completely disagree most of your points. Health is the most effective item early combat stat in the game, not armor. Resistances are more effective in the late game when you already have health.
By the time one can afford Thornmail, 100 armor is going to provide the same %increase in effective HP as an item like Warmog's.
The reason HP is better early on is due more to most champions dealing some level of mixed damage.
After about 60 armor the returns on the resistance diminishes without health.
After 60 HP the returns on HP diminishes without resistances.
Both stats scale in a largely identical way.
1000 HP will allow a champion to survive 1 1k rotation.
2000 HP will allow a champion to survive 2 1k rotations.
3000 HP will allow a champion to survive 3 1k rotations.
100 armor will cause a champion to take 1/2 damage
200 armor will cause a champion to take 1/3 damage
300 armor will cause a champion to take 1/4 damage.
The more you get of either, the lower the increase in your total power.
In general this makes getting both at once better than prioritizing just health or just armor, though with very affordable and very high values in either health or armor nothing is really given up pursuing 1 for 1 major item.
Also, it's worth noting that HP doesn't improve any flat health gains or healing, where 50% damage reduction effectively makes a 10 HP heal a 20 HP heal. Healing potions, HP/5, HP shields, Max HP increases, life steal, spell vamp: anything that restores HP is always improved by armor vs. a physical-damage opponent.
The inverse effect is true vs. life steal: 50% damage reduction turns an opponents attack from one that does 100 damage and restores 10 HP per hit to one that does 50 damage and restores 5 HP per hit, making armor extremely good at countering life-steal.
Thornmail offers no health and a weak passive that scales off a user's bonus armor. It doesn't offer CDR, mana, or health. This makes it a more effective late game item (when you have more bonus armor) than an early game item.
The passive is oppressively strong in-lane.
It's mostly feels weak because it does not scale well into late-game.
Also: the passive scales because it's the kind of thing that should scale, not because it's something meant to be amazing late-game.
Auto-attacks scale harder and faster than anything else in LoL. Thornmail's passive does not scale very fast, especially seeing as the initial 100 armor offered by Thornmail is the biggest potential scaler the item can get, where most decent armor items only offer around 50 armor.
So when auto-attacks are going from 100 to 200 to 400 to 800 to 1600 DPS, Thornmail's passive goes from 100 to 150 to 200 to 200 to 200 (those MR items).
Thornmail's damage reflected vs. damage taken also gets worse when armor penetration comes into the picture, seeing as reflecting damage before it's mitigated by armor is great when armor is mitigating a lot of damage and not great when an opponent is shredding through most of it.
Obviously much better early-game.
It is not a rush item in the top lane, except maybe verse a Vayne.
Tryndamere and Gangplank are much better champions to build it against, though the likes of Quinn, Draven, Rengar, and Aatrox aren't bad.
Most top-lanners that Thornmail is good against are fairly uncommon, though watching GP crit himself in the face due to thornmail is still pretty satisfying.
FH would still be better rush item since it offers high armor as well as mana, cdr, and a great passive.
That really depends on the matchup and what your opponent is trying to accomplish top-lane.
If they're going to back-door 24/7 while trying to beat your face in through drawn-out fights, getting an item that offers power in short bursts and provides a strong aura within teamfight situations isn't the best way to go. Ditto for trying to split-push with sustained DPS yourself. Thornmail is much better in those situations.
Thornmail's main falling is that it's much better in 1v1 situations than 5v5 situations, which makes items like FH the better item to get in the long run for a range of tanky units that plan on winning a game off the back of a 5v5.
Thornmail is an item without much of a place except in the late game if they are mostly AD
Late-game AD's usually have enough power to crush tanky targets regardless of how much they bulked up, and it's a bad idea to get focused by them at all. Thornmail isn't really good in that situation, especially seeing AD heavy champions will hardly be tickled by the passive at that point. It will set their life-steal to 0%, which does have tactical applications, but that's about it as far as the passive goes. Anyone that seems useful with thornmail late game would have been useful with any other armor item.
Early-game: reflecting 15% of damage taken (prior to it being mitigated by things like your armor) and dealing damage equal to 25% of your bonus armor means ~34% of the damage you actually take is reflected back + 25 damage.
100 armor also pretty much doubles your HP, taking an 800 HP 30 armor (1040 EHP) champion to a 800 HP 130 armor (1840 EHP).
Even with 170 AD, you'd be reflecting half the AA damage taken back to an opponent while also having an 80% boost in overall HP, leading to a situation where an auto-attacker would need 11 auto-attacks to kill someone in thornmail: and kill themselves with 13 auto-attacks. If the champion wearing thronmail does anything but AFK, that's an extremely easy lane to win and a very dominant early-game.
Doing enough damage to take out ~84% of your opponent's HP with Thornmail alone early? Strong. Very Strong.
Doing enough damage to negate your opponent's life-steal with Thornmail late and maybe enough to take out ~5% of their HP? Weak
Thornmail is not in any way a late-game item.
It's an early-game item.
It's an item to get early-game to make a carry miserable early-game while you prevent them from farming CS, push them around, and generally prevent them from reaching end-game status.
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) and they don't stack. ADC's have a lot of options but and all AD comp, unless they are fed and end early will not scale effectively late.