You need a starting point as a baseline which you can then modify based on magic and other factors. The real world is good as that starting point.
Healing is more prevalent in a fantasy world, but how much it helps with disease is very setting-dependent. You could easily have a setting where the nobles and wealthy merchants aren't going to die of the plague, but everyone else will at real-world rates. (A setting where no one dies of disease is also possible, at the other end of the scale.) Also, most fantasy worlds have considerably higher levels of danger with undead, reavers of various types, etc. So I don't know that the net adjustment should be upwards.
I do think it's reasonable to have fewer small indefensible villages in a setting where most of them will be destroyed by wandering monsters. And that means larger villages, probably more people in towns, possibly more people in cities (depending on the ability to supply them with food).
Healing seems important but you might get more bang for your power points by casting spells like Speed Growth II (10x plant growth rate for 1 day in a 100'R). Blessed Fertility on Sacred Land in RMU Treasure Law gives +50% harvests in a consecrated area (which could be large).
As for the critique, S. John Ross wrote:
Some Historical Comparisons: Medieval France tops the list, with a 14th-century density just upwards of 100 people/sq. mile. The French were blessed with an abundance of arable countryside, waiting to be farmed. Modern France has more than twice this many people. Germany, with a slightly less perfect climate and a lower percentage of arable land, averaged more like 90 people/sq. mile. Italy was similar (lots of hills and rocky areas). The British Isles were the least populous, with a little more than 40 people per square mile, most of them clustered in the southern half of the isles.
Lyman Stone agrees about 14th century France. And "By 1450, max densities range from 110 in Belgium to 90 in Italy to 74 in Germany." So basically the difference seems just to be that S. John Ross has chosen a later period and Lyman Stone has chosen an earlier one. It's not that one is right and the other is wrong, it's just that they are trying to model entirely different things, and you should pick which is more appropriate for your setting. That's pretty much true throughout what he wrote, he's dismissing later things everywhere, e.g. Italian mercenaries, simply because they are later period. So if the later period is a better match for your setting, you can just read the parts he calls abnormal and use them.
I also think Lyman Stone is a bit off about dismissing the number of villages just because they are too many to name. Who cares? You don't need a name for every single village any more than you need a name for every single person. You name the ones that matter.
