[...] some kind of "golden path"
[...] I have seen about 5 castaway gals in all of these 18 months.
[...] every enemy needs to be stunned
[...] What are you using for the first two months? Nothing but handles?
[...] so these actual values should be 1st month income: 1383438, 2nd month income: 2938138
[...] What version of the game did you do all this on?
Well, no, it's not a golden path or anything. Just maximizing your production. Production -> Money -> Bases -> Research
Bro, you get (on average) 4.3 castaways in the first month (not including events).
3 from Roving Bands (100% mission spawn in the 1st month, 30% chance next 7 months).
Shipwrecked Gal (76% for mission * 61% for gal = 46%)
Red Lantern Villa (84%)
Only stun the enemies that are worth bringing back alive. For that reason most bandits and necropirates can be killed.
Handle is one of the worst stun weapons, even worse than Bare Hands. Don't ever use it. You should get a couple of Stun Batons from the first three Academy Airbuses. But even if you don't, you have a 50% chance to spawn Academy Outpost where there's plenty of stun weapons. Right, what are some good early game stun weapons?
For melee, for untrained units I'd recommend Shepherd's Staff because of high accuracy and guaranteed damage. Cattle Prod against armored targets. Once your skills are trained, Stun Baton becomes better. Leather Whip is also a safe choice if you're good at throwing.
For ranged, there's not much choice. Domestic Shotgun Rubber Slugs. Harpoon Gun once your accuracy is high. Again, obvious to say this but, do night missions so it's easy to get in melee range. Use Fire Extinguisher for smoke in day missions. Melee enemies from behind...
Doing the math... 1384k jan - 942k maintenance + 335k countries + 967k score bonus + 2157k feb - 1969k maintenance + 375k countries + 3354k from 10072 score (5550 from research) = 5661k. So in total, $5.6M spendable money was generated, which was enough to build up the 1st base and start a new base.
That was in L4. I've upgraded the version along the way, added the missing ninja bases, etc, but I can't say that the early game changed much.