Well…for starters I’d hardly call it the “Titan Quest engine” at this point given that we’ve rewritten significant portions of it and implemented all sorts of upgrades and new features while making Grim Dawn. The pathing system isn’t even the same as TQ used. For all intents and purposes, this is the Grim Dawn engine.
And no, hacky solutions are what designers do; until their hacks no longer work, then a programmer gets involved to make the engine do what the designers need (or the designer hacks make a programmer cry, then the hacks get replaced).
The final solution is far from “hacky”. I was not the one who wrote the code, so I can’t go into explicit details, but I imagine vector calculations were involved to determine a valid pathing solution and whether the targeted destination is within a valid pathable range so as to avoid pathing over inaccessible areas or areas too far away to reach normally.
The core of it was getting teleport to work and then working backwards from there to create the other variants and behaviors. Adding a whirlwind-type skill was a different beast entirely since it involved a channeled attack+move command, which is unlike any other skill presently in the game. The channeled behavior from skills such as Aether Ray served as a starting point.