Like looking at the weapons xml, the bash ability is duplicated under every blunt weapon. I feel like if it was up to me, I might have abstracted that to having various weapon types and then having a weapon be able to attach any number of types to it. Oh well, at least it's doable, if not easy...
They at first had the blunt weapon damage in the previous iterations, then added these bash abilities later to all blunt weapons to make distinction between the weapon types. Before not all blunt weapons had the bash ability.
I've not tested the race config trick since I don't play with the tech tree too much, but it should work out, without overwriting the tech-tree and the race configs beyond that tag (I did a simple change to them with my tent building style mod). I think adding techs is no problem, it is changing the pre-existing techs that leads to the problems.
The compatibility with CoS, has to do with quite a few added features that CoS uses, like the background stats base mod he uses. This adds the necessary tools for his weapons and abilities to interact with all the races. So, when a mod adds a new unit, modular mind you, it is not compatible unless you add the appropriate tags to the unit, so CoS knows what to do with these units.
The game is very modular, although there are some quirks like the tech tree. For example changing spells, when you change a spell from the game it keeps it's old casting cost, but everything else can change with the spell. Just something to be aware of in the object insanity of modding the game.
