Why the Luggage is frequently separated from Twoflower: Losing your luggage is an integral part of being a tourist.
The Wyrmberg is a huge peak that, somehow, stands upside-down. A later novel's description of Lu-Tze's bonsai mountains may explain this, as Discworld mountains evidently have a morphogenic field that's projected downward from the bit of stone at their very tops. If Lu-Tze can harvest those particular bits, and grow them on dishes to create miniature mountains — complete with diminutive glaciers, streams, forests or even vulcanism — then why couldn't one of those tip-of-the-peak bits that was accidentally turned upside-down grow an entire mountain in that orientation?
Death's apparently-spiteful killing of a man in the street seems completely out of character ... unless, that is, you stop and consider that Death specifically says he wasn't there for Rincewind. Presumably, the fish merchant who keels over from a heart attack is the person he had been looking for, and the fact Death snarls while "collecting" him just means he was irritated that his conversation with Rincewind had put him behind schedule to do so, not that he'd murdered someone in a fit of pique.