Yes a grinding meal is old technology. The kind used in daventry is the sort with a water wheel turning two stone wheels. That kind itself has been used for a long time.

I didn't say it was impossible just that we don't know how manhy people actually live in Kolyma. Any invasion would have to come from eastern Kolyma, if an invasion was to occur. We certainly don't know how many live there.
There are not enough people in western Kolyma to take over a nation as large as Daventry. Just a few monks, an old lady, Grandma, big bad wolf, Little Red riding hood, Hagatha, a Dwarf, and who knows how many undead... But certainly not enough to make invasion plans. Especially with Neptune preventing the vampires by leaving by sea.
Daventry is also protected by invasions by use of its Magic Shield. As long as the enemy isn't some kind of magical creature such as a three-headed dragon, or magical tempests turning everyone to stone, or powerful wizards like Mordack, or Telgrin.
Any nations that would try to attempt to take over Daventry would have to go through the local countries around the nation first. Consequently the other nations put up walls around Daventry when the dragon reached it, so that it wouldn't leave the country and damage their lands further.
Daventry in all of the games other than MOE, is shown to be in a valley in mountainous region towards the middle of the continent. So it's location is actually kind of remote, except to other countries nearby it, living in similar circumstances.
Apparently for Daventry to even reach its fleet it would have to travel through other nations, though perhaps it can access the sea by river.
In MOE somehow Castle Daventry arose on top of a mountain, and half the continent is shaved off placing it closer to western sea. But as Derek Karlavaegen explains things, the world is always reinventing itself and is in some kind of magical flux. The strange changes could be other changes caused by the catclysm but who knows.