Superpower isn't legal with Multiscale*. TRE/R Inanimate made a list of all the Subway pokemon: http://members.shaw.ca/teamrocketeli...ubwayData6.txt
(Probably there's a nicer looking list somewhere else? I dunno), so you can see which pokemon has which EV spreads.
Basically, if you do the calculations from that list of pokemon, you'll see that +1 Fire Punch is basically same or better in most cases even vs Ferrothorn (it's probably OHKO on Scizor either way - I didn't bother doing calcs) unless Multiscale is really that important to you (but as mentioned, without roost, you really just have one turn of Multiscale).
*Maybe it's just me, but Multiscale without roost and without Dragon Dance is not that useful. The main selling point in Multiscale is the half damage at full health. With Roost, Dnite can reactivate it often, even stalling out ice moves (not that that's recommended due to freeze probability). In Dragon Dance's case, an offensive Dnite (without Roost) can survive nearly any hit (or 2) with Multiscale as it dragon dances (once or twice) and sweep. Without either of those, Dnite isn't going to make much use of that awesome Multiscale of his. Hone Claws does not work as well in DD's case since speed is not increased and Dragonite is not that fast even with Jolly/Hasty. Extremespeed remedies this speed problem and as such makes it a good use of Multiscale when combined with Hone Claws... which was why I suggested Adamant instead of Jolly, because that takes the most advantage over Multiscale ... if you have been following my train of thought.
The second paragraph I typed which you didn't understand was talking about the fact that Fire Blast is mainly for steel types coverage, but +1 EQ actually hits non-4x-fire-weak either just as hard or harder than Fire Blast, and by not going mix, Dragonite can have better defense. Sorry my brain was wondering around everywhere during that paragraph.
I know I commented a bit on this already, but just to make sure you understand what you're saying:
"I know i can use Dragon Dance with Fire Punch, fine, but Scizor and Ferrothorn, the main threats, have a SDef lower than the Def, and also Forretress and Skarmory, even if they have Sturdy, so it can't be 1HKO. As Fire Blast has much more bp, and is the main reason to use Hone Claws, i'd prefere to keep it"
- I talked about the first part already, but just to reiterate:
--> There are 4 Ferrothorns in Subway. All of them are 2HKO'd by no EV Fire Blast and Fire Punch. Two of them are +Sp Def (and actually have higher SpDef than Def)... those two are OHKOd by +1 Fire Punch.
--> Your reasoning for Fire Blast vs Forretress goes the same for Fire Punch. Without Sturdy, Fire Blast OHKOs Forretress (didn't do calc, but probably it does). However, it'll be a 2HKO because of Sturdy. Fire Punch does not OHKO Forretress, but will 2HKO. So, Sturdy makes both Fire Punch and Fire Blast 2HKOs. Without Sturdy, Fire Blast has an advantage. With Sturdy, Fire Punch and Fire Blast are the same.
EDIT: Ok, well, I'll maybe give a point for Fire Blast on this one, because there IS a Custap explosion Forretress. Fire Blast + ES will KO the Forretress before it custap explodes (or custap spikes, whatever), whereas +0 Fire Punch + ES does not KO, so at no Hone Claws boost, Fire Blast + ES has 85% to KO Forretress without taking any damage (assuming Forretress lays hazard on first turn ... which it will). That said, facing Forretress (especially that one, it's best to Hone Claws first turn regardless of Fire Blast/Fire Punch anyway... no point not trying to set up on something that can't hurt you. Sorry for triple negative. I.e. should set up on something that can't hurt you. And... with 1 Hone Claws, Fire Punch and Fire Blast are on even grounds again as Hone Claws+Fire Punch+ES and Hone Claws + Fire Blast + ES both are guaranteed KOs. ...
--> For Scizor, Fire Punch >= Fire Blast, and here is why. +0 Fire Punch apparently has 99% chance to OHKO both of the non-Occa Scizors with 244 Attack EVs (with 252 Attack EVs, OHKO is guaranteed). So, Dragonite OHKOs Scizor even when it did not have any Hone Claws, whereas with Fire Blast, it will only OHKO 85% of the time due to accuracy. Now, what about Occa Scizor (since half of the Scizors have Occa in Subway)? Occa Scizor is not OHKOd by Fire Blast. Fire Punch is still a guaranteed 2HKO obviously. Now Fire Blast only has 0.85 * 0.85 = 72% chance to 2HKO due to accuracy. With Hone Claws, both moves are guaranteed 2HKO, so if Dnite has a Hone Claws already when facing Occa Scizor, both moves are equal. (I.e. in summary, with no boost, Fire Punch > Fire Blast. With boost, Fire Punch = Fire Blast, so in general, Fire Punch >= Fire Blast).
--> Regarding the last sentence in that quote... I'm no expert in damage calculations, but I do know that damage is based on AttackStat * Base Power. After 1 Hone Claws increasing Attack and Accuracy -->
Fire Blast is: 120 (Dragonite's Special Attack at Level 50 no EVs neutral nature) x 120 = 14400
+1 Fire Punch is: 185 (Dnite's Attack, 244 Attack EV, lvl 50, neutral nature) x 75 x 1.5 = 20812, which is 44% higher.
The fact is that at +1, Fire Punch is effectively 112 base power vs Fire Blast's 120. Yes, Fire Punch is still lower base power, but backed up by a much higher attack stat.
Not saying Fire Punch is for sure the way to go or better, but the reasons you gave do not really support your argument very well. That's all.