If I'm following your notation right, it looks like you mixed up Flamethrowers and Miniguns.

high numbers of soldiers,

 low numbers of soldiers,

• A horrifying number of Tyrants,
• A large quantity of Scarabs and Abominations, and
• A below-par-given-they-showed-up-at-all-but-still-significantly-above-zero count of Crawlers and Venompedes.

• Artillery is the optimal counter for Tyrants.
• Miniguns are very good at handling Scarabs (to the point that bringing more than one would likely be overkill), and pretty useless at most handling most other xenos (to the point that bringing more than one would likely harm our chances).
• Lances are good counters for anything which isn't a Tyrant or a Scarab. (And also not-terrible vs Tyrants)
• Torpedos are slightly better than Lances when facing Abominations, and only slightly worse than Artillery when facing Tyrants.
• (As far as I can tell, the other four weapons aren't worth considering.)

8 Soldiers: 3 Artillery, 2 Lances, 1 Minigun, 2 Torpedos.

(My model says this gives me >99% chance of survival, but also says that about just bringing one of every weapon. We can be more daring!)

7 Soldiers: 3 Artillery, 2 Lances, 1 Minigun, 1 Torpedo.

(My model says this gives me ~95% chance of survival.)

6 Soldiers: 2 Artillery, 2 Lances, 1 Minigun, 1 Torpedo.

(My model says this gives me about a 2/3 chance of waking up the next morning.)

5 Soldiers: 2 Artillery, 1 Lance, 1 Minigun, 1 Torpedo.

(My model says this has slightly worse odds than a game of Russian Roulette with five bullets loaded.)

4 Soldiers: 1 Artillery, 1 Lance, 1 Minigun, 1 Torpedo.

(My model says this almost gives me an entire 1% survival chance.)

7 Soldiers: 3 Artillery, 2 Lances, 1 Minigun, 1 Torpedo.

• Duels between a Tyrant and an Artilleryman always end well.
• Duels between a Tyrant and a Minigunner, Phaser or Flamethrower always end badly.
• Tyrant vs Artilleryman 2v2s . . . don't happen, ever. (Turns out the quartermasters do display some nonrandom behaviors, and one of these is a bias towards weapon variety.)
• 2v2s involving two Tyrants, an Artilleryman, and someone who'd lose a 1v1 against a Tyrant . . . end well pretty much exactly half the time, regardless of which [MPF] is used.

This is what we'd see in a turn-based fight where humans aggressively heroically always take the first move, and the xenos move randomly. The Artilleryman caps a Tyrant every time; the remaining Tyrant then picks a random human to squish; they pick the dud half the time; we get the coinflip we see.

I find out that there are 2v1 fights between two Tyrants and a lone Artilleryman, and these have the exact same 50% win chance; the dud isn't even useful as a decoy; my hypothesis is falsified.

Absolutely nothing.

• There's a random element. (Existence proof: 16079 and 17759 were the same fight but we only lost 17759.)
• There's an implicit chrono effect: It looks like this war has been developing not necessarily to our advantage. (Luckily it seems like this is probably 'just' enemies outnumbering our troops more frequently in later rows, and not anyone actually getting better/worse at their job.)
• The number of troops sent scales with the size of the enemy forces, making inference trickier; however, I haven't seen anything contradicting the hypothesis that loadouts are decided by throwing darts at a board.
• Specific weapons counter specific enemies: in particular, the Minigun is usually pretty lousy, but drops Scarabs like flies.
• I expected to find synergies between weapons, and didn't. I did, however, find some antisynergies: Miniguns and Flamethrowers are hella redundant (presumably because they're both anti-Scarab bugspray), and the [MPR] set all clash with each other ("Why do you need Gun? You already have Gun!")
• Guaranteed victories seem possible. (A single soldier with a minigun can perfectly-reliably survive 5 Scarabs, but not 6.)