Double Hit breaks subsitutes as well as doing damage in one turn, and breaking sashs.
I don't really see the use of Double Hit over Return at all. Double Hit's total base power is 105, and Return's is 102, so in basic damage / accuracy terms, Return is far preferable because of its guaranteed hit (think Flamethrower [95/100] and Heat Wave [100/90] - anything with access to the former wouldn't consider the latter). Secondly, you're using Double Hit to break Substitutes; I'd say the three most common users of Sub in UU are Mismagius, Rotom, and Hariyama. Double Hit doesn't affect the first two so being able to break subs is irrelevant. And against an 80/252 Hariyama, you do
22.3 - 26.3% with the first hit of a LO boosted Double Hit, which means you have only a 22.5% chance to actually break the sub. Your final point, that Double Hit breaks sashes - well isn't that the point of Fake Out?
Long story short, don't be tempted by Double Hit. It sounds good on paper only, and its 90% accuracy will let you do far far more than its sub breaking ability will make up for.
Fire Punch is for registeels, Specs Yanmega (which it OHKOs) and Roserades.
STAB neutral Return: 102 x 1.5 = 153 base power
super effective Fire Punch: 75 x 2 = 150 base power
So against Yanmega and Roserade, Return actually does more. Also, your claim of OHKOing Yanmega:
LO Fire Punch vs 0/0 Yanmega:
64.5 - 76.0%
Not really an OHKO by any means. Personally, I'd much rather have Brick Break as the super effective move for hitting Registeel, since it also hits Regirock and Omastar super effectively too, something that Fire Punch doesn't help with at all. The only other reason you might want Fire Punch would be if you wanted a move that would cover both Registeel and Ghosts to an extent, in which case Fire Punch could be used as a sort of compromise between Brick Break and Pursuit / Payback.