Holy triple post, Batman!
Some places really are much harder to farm than others, if that's what you were disagreeing with.
Temperate-climate farming requires more foresight than tropical farming, for instance. Harvestable grain grew wild in Anatolia 8,000 years ago, and replanting was simple, but successful grain farming in far NW Europe is a lot tougher. Dairy farming requires different skills again (and a lot of hard work), and so on.
I was objecting to the statement that "the levels of skill involved in porterage and subsistence farming were comparable"--certainly the level of physical effort involved in digging holes, picking plants, etc. is similar, but being a farmer is much more than just sticking seeds in the ground and pulling up what grows there.
Your approach is incompatible with Malthusian theory because you have farmers producing a vast surplus of food - 1sp/day is enough food to eat, 5sp for a family, but you're letting them roll Profession skill and produce ca 10-15gp worth of food each week! In this society only a minority will work the land, and by definition they will have the same income as craftsmen, since they all roll Profession skill to generate the same income. You have created something more resembling a 19th century western-European or north-American economy, not Renaissance.
1) Malthusian theory ≠ "everyone is a subsistence farmer." His theory simply states that the population's demand on resources will inevitably outstrip their ability to produce it. That doesn't mean that on an individual level every farmer barely makes enough food to feed himself, it means that as time goes on Σ [total food output] < Σ [total food consumption] barring mitigating factors.
2) Farmers aren't producing 10-15gp worth of food each week, they're producing 10-15gp equivalent of
wealth each week. That could mean he produced surplus food, or it could mean that the farmer fixes a broken fence that he'd otherwise have to pay someone to fix, or that adventurers sweep into town needing more supplies and he sells it at a markup, or whatever.
It doesn't matter how good at farming they are (and how good you need to be varies a lot by environment), what matters is how much income you generate, in food to eat and sell. Those skilled modern subsistence farmers are producing around $1-2 a day in exchange terms, below even the 3e 1 silver piece standard. Although if their markets weren't flooded with cheap food from the USA and other developed countries, their produce would admittedly be worth substantially more, and on PPP is probably nearer $10, about 1 3e sp.
I wouldn't say 1 sp is about $10; it's harder to nail down prices than that. Take a look at the price of commodities today and compare to PHB prices. A bushel of wheat goes for
about $319, or 319/60 = $5.30 per pound; 1 pound of wheat in D&D is 1 cp. Silver is going for
about $36 per ounce, or 16*36 = $576 per pound; 1 pound of silver in D&D is 5 gp. Goats can cost
between $100 and $300, so let's sat an average of $225; in D&D, a goat is 1 gp. In one case, we have 1 sp = $53, in another case we have 1 sp = $57...and in a third we have 1 sp = $22. Trying to equate D&D currency to real-world currency doesn't really work too well.
And again, the Profession rules don't mean you actually gain that many gold pieces at the end of the week, any more than a PC actually gets Xd4 gold pieces to start his career. When the subsistence farmer comes out with a net of $1-2, that's after he "buys" his meals, after he "buys" repairs to equipment, and so forth. Unskilled laborers make 1 sp per day and a day's worth of poor meals is 1 sp, so any subsistence farmer who can eat enough to survive and still have even a dollar left over to put away for a rainy day is coming out ahead of the unskilled laborer in income.