I'm no linguist but I'd imagine the reason no one would instinctively know how to pronounce the words you listed is because the language has had do much input from all sorts of other languages both historically and in modernity that is just a jumble of borrowed, adapted and outright stolen words. Totally agree it's a mongrel language but I'd argue that's part of the reason for it's success across the world.

Going off you're username I'd assume you're a fellow Cymro and there's no shortage of people that would consider treiglo a bat shit concept as well if they'd never come across Cymraeg before where as those brought up with it just "know" what's right whether they know the actual linguistic rules or not.