I'm not exactly sure what your argument is here to be honest. Let me preface by saying I've never played any of the original Baldurs Gate games so I can't judge what they did. But as for the source and the larva being contrivances to force mechanics into the plot of their respective games...yeah, and? Games do that all the time. Dark Souls is full of contrivances needed to get mechanics into the game; dying and being reborn, the way you use souls, humanity and the way reversing hollowing gives you extra powers. In Mario, all those power-ups are contrivances to get mechanics into the plot. In fact it's more accurate to say that they're contrivances to explain mechancis in a plot, since the mechanics would in most cases be there in some form no matter what, since the devs want those mechanics in the game.
As for your earlier point about the Bhall heritage never specifically holding you back or requiring you to deal with it to unlock your ability to play the game, I want to address the implication that the source collar and larva do do those things. I feel the need to point out that doing something about the source collar isn't *unlocking* your ability to play the game, it's part of the story of the game that you're playing, as is the larva. That would be like saying becoming a Gray Warden in Dragon age is something you need to do something about in order to unlock your ability to play that game.
