Hi, that's a tricky, but very interesting, question, which answer depends on a lot of variables
First of all, I consider Mystara as big as Earth, with slightly less than 60 millions square miles of land area, 115 millions square miles underwater and some more square miles in polar areas.. the HW would around 14 millions square miles of land area and 26 square miles underwater. (see viewtopic.php?f=22&t=8327&hilit=circumference+equator
All that matters to calculate a consistent population density of the planet.
The human population of Earth is estimated from 5 to 20 millions before agriculture, then 100 millions in 500BC, 300 millions in AD 0, then down 30% for plagues, barbaric invasions and the fall of the roman empire, then rising to over 400 millions in 1340 but down to 350 millions after the Black Death, half a billion around 1550AC, 600 millions in 1700, 900 millions in 1800, 1600 millions in 1900, 2 billions around 1925 and then to a meteoric rise up to over 7 billions now.
That's also important because if Mystara during Blackmoor times was a highly developed technomagical society we should expect a high population density, but the cataclysms you mentioned could have killed up to 90% of the total population.
Now the population density of Mystara is another problem... Zendrolion's Mystaran Demography Book gives a figure of 14 millions inhabitans, humans and not, from Wendar to the Sea of Dread and from Sind to Thyatis, in a surface of slightly more of 1.1 million square miles. The KW could be a bit more populated than the rest of the world, so let's assume an average of 10 intelligent creatures per square miles in all Mystara (the KW is 12,5). That would give a total mystaran population of 600 millions intelligent creatures (including humans, demihumans, humanoids, faeries, lupins, rakastas and other intelligent beings). A figure I think it's consistent with a world that has many developed nations (of renaissance and "late empire" level) and magical healing (that should greatly limits the effects of many plagues).
The Hollow World should have a lower population density, maybe half, but still it could have up to 70-100 millions intelligent inhabitants.
There are also underwater creatures and kingdoms.. let's assume that in the sea the population density is much lower, maybe one tenth of the surface, yet the mystaran oceans could still be the home of 100-120 millions intelligent creatures, and another 25 millions in the Hollow World seas.
There are Aerial creatures too (like faedorne, faeries, cloud giants and air creatures), and I think they could amount up to 5-10 millions intelligent beings between the outer and hollow world, even with a very low population density.
To conclude, what could have been the total population at the height of Blackmoor times? With a highly technomagical society I would go up to 3 billions... if the cataclysm, the vulcanian supervolcano and the wasting killed 90% of the population, in 2300BC the Outer World population could have been around 300-350 millions, a bit more than half of "modern" Mystara.
Add at least, IMHO, 50 millions intelligent creatures in the oceans and maybe the same figure in the Hollow World (considering that three of the bigger and more populated empires, Milenia, Nithia and Azca, and many other cultures were not there yet).
Ok, that's the first day since long time with less work and I'm exploiting it..