Hey nice write up, but I'm gonna disagree I think with heatran. It can absorb rock attacks nicely, and with the right investment can check diancie (which note, that is something Darmanitan can also do), but on a usually hyper offensive team, there isn't a lot of room, and there's also the fact that it has no recovery, which really hurts it's role as a wall. I'm gonna say B rank for that.
As for Infernape, I would slash Thunder Punch on some of these sets. Very helpful against water and flying, and it can check gyarados prior to boosting. You could even slash HP Ice on the LO set, since it sits at a great speed tier, so it can potentially check Chomp, Landos, ans even certain dragons. Nice write up otherwise
I have to disagree with your tiering placement of Heatran, and definitely agree with Zarif 's placement of A rank.
Like Zarif said, Heatran is one of fire's only checks to Mega Diancie, and honestly that alone gives it a case for A rank. Aside from Heatran's awesome typing and defensive capabilities, Heatran also makes it to where the fire user doesn't have to run Torkoal, and can run a more hyper offensive team.
Here is the Heatran set I like to run:

Heatran @ Air Balloon
Ability: Flash Fire
EVs: 252 SpA / 4 SpD / 252 Spe
Timid / Modest Nature
- Fire Blast / Magma Storm
- Flash Cannon
- Taunt
- Stealth Rock
This set allows you to outspeed and disable a large portion of hazard setters in the tier with Taunt, replace Torkoal as the SR setter, hit hard with respective STAB moves, and check Mega Diancie with Air Balloon + Flash Cannon. Earth Power can also be run on this set, but I don't like it because all of these moves are almost necessary. (But if you were to run it, I would recommend running it over Taunt.)
Although you lose access to Torkoal's Rapid Spin, Taunt + Defog Mega Charizard Y deals with hazards fairly well, and this set can make your team much more offensive.
And by the way...
252 Atk Darmanitan Earthquake vs. 0 HP / 0 Def Mega Diancie: 214-252 (88.7 - 104.5%) -- 25% chance to OHKO
While...
0 Atk Mega Diancie Diamond Storm vs. 0 HP / 0 Def Darmanitan: 524-618 (149.2 - 176%) -- guaranteed OHKO
And...
252 SpA Mega Diancie Earth Power vs. 0 HP / 4 SpD Darmanitan: 368-434 (104.8 - 123.6%) -- guaranteed OHKO
I don't want my Mega Diancie check to only have a 25% chance of beating it.
And if it doesn't Mega Evolve, you don't even have a chance to OHKO...
252 Atk Darmanitan Earthquake vs. 0 HP / 0 Def Diancie: 162-192 (67.2 - 79.6%) -- guaranteed 2HKO
While...
0 Atk Diancie Diamond Storm vs. 0 HP / 0 Def Darmanitan: 348-410 (99.1 - 116.8%) -- 87.5% chance to OHKO
Personally, I would rather have this as my Mega Diancie check:
252 SpA Heatran Flash Cannon vs. 0 HP / 0 SpD Mega Diancie: 484-576 (200.8 - 239%) -- guaranteed OHKO
While...
0 Atk Mega Diancie Diamond Storm vs. 0 HP / 0 Def Heatran: 154-183 (47.6 - 56.6%) -- 32.8% chance to 2HKO
tl;dr: Heatran has many attributes that are really beneficial to Fire's type as a whole, making it A rank for Fire.