So if we just invent a whole different story than the one presented, then him being a blabbering fool that sounds straight out of a comic book version of Twilight is actually on purpose to lull you into a false sense of security and totally not descriptive of him, and him having a very powerful class is actually him being really powerful as a character even though he's lacking most of the vampire spawn abilities that the monster manual is telling us he ought to have. Right.
I mean, technically we can do this whole "the game presents this, but it could just be a ruse!!" thing with all of the origin characters. We can do this with the entire plot, even. Up can be sideways, and down can be off somewhere to the right. I don't mind your story, and I definitely prefer it to what we have, but it isn't actually in the game yet and it might never be.
But if Larian decides to actually make him more vampire spawny and less of a lame squid on dry land then I reckon it would fix his issues for most of us. If they do. If they don't, if they just keep going down the track we're seeing, then that would make him a vampiric teenage-version of Volo that we're somehow meant to take seriously.
Also, isn't Shadowheart just as stealthy as Astarion? She doesn't get backstab and cunning actions but that hurt and hide tactic is her bread and butter too, if necessary.