[Fingers]

You're missing the point of (1). I know of very few games that allow their "gameplay mechanics" to be changed in mods, and even fewer mods that significantly change them even if they could. Half-Life Rally is the last one I can remember. For example, by your definition Doom wasn't a moddable game which is obviously false.

As for (2), it's up to the modder to test their mod and make sure it doesn't crash. I help the modders by answering their questions, such as by telling you the maximum cargo hold size is 20; and by updating the modmakers' guide if I ever have time. I am also working on better error reporting, but while residing on the prime material plane with its linear time continuum it's just not possible to catch every way a modder can crash the game.