Steam tech support? Never knew they had one; might be helpful if it's a Steam issue. Paradox tech support would probably be more helpful, but that forum is hidden until you register the base game. I suspect his primary issue (loading any saved game) is a problem with the install or configuration, but they deal with these issues all the time. I assure you/him that saved games are loadable.
On the separate issue of patch compatibility with saves made under prior versions: at the risk of coming across as a Paradox apologist, patch compatibility with save games means different things to different people. Unless my memory fails me, the vast majority of recent patches, including the 5.1 DW are generally savegame compatible, if you're just talking about the ability to reload a saved game. Yes, they make a point not to promise compatibility, and warn you that wonky things may happen. But if you want to continue a saved games under the new patch, it's generally quite doable (and I've done it many times). It's only when they change the way something is stored in the game files that saves are actually unloadable (rare).
However, since each patch changes the rules, to some extent or another, most people find it kind of pointless to continue a game started under one set of rules, with a different set of rules. Even a slight change to ... say ... naval maintenance costs or length of time for a horde to annex a province has a big influence on how a game will flow. So your savegame that has England hamstrung, or the Golden Horde occupying half of the HRE might be highly improbable, under the new patch. So most people prefer and recommend a fresh start.
Is it possible to make things so compatible that a save could be completely compatible? Sure. Is it desirable? No. You'd essentially have to re-run the game from the start under the new set of rules. There's just no realistic, cost-effective way for a programmer to undo the myriad ripples that such changes would have on a game. Not only would the costs be prohibitive, for miniscule gain, the performance hit for storing and tracking all the decisions and history, that would have to be undone/redone, would be unacceptable.