Round table

If on week 1 for Inferno you could grind out 50 demons in at least 20% of games, people would buy it for crazy cash, ignoring all sorts of schmucks like Brogh, Gosha, etc.

External dwellings and empty secondary heroes are radically insufficient for this. You can just calculate by hit points — 50 demons is 1750 HP. The starting army is about 900 HP. An average edible secondary hero is another 150-200 HP (if not built up). If you build them up, then a bit more.

Plus an adjustment for the variance in grinding and losses on neutrals (almost inevitable with such a puny army as Inferno's). I'd roughly estimate this variance at 20-50% depending on the spawn. So all in all, gathering 50 demons by week 1 may be possible, but only in some megabonanza, or through a Pandora's box with demons.

Almost guessed it, that's the essence of the strategy, and it shifts from what we're used to towards grinding itself. If you're playing on SM-c, say, spawned at the top, you usually get many external dwellings from the random spawn on W2, plus you can clear both secondary heroes on week one, virtually without losses, and there are also secondary hero external dwellings (often), plus you need to build secondaries up to level 3-4, and at home 1-5 in the Citadel/Castle. So do the math, where I was grinding that fifty :)
Of course, you won't always get fifty, but more often you'll have at least about 40 by day 1-1-7, and you can even muster fifty if you get a bit lucky with the layout and set the strategy correctly...
And since few people know about this and few know how to do it, they don't believe in it and underestimate this town on random, and don't sell it for "crazy cash" :)