I gotta say, I'm a little bit stumped on this one. In your OP you are saying that the acetaldehyde smell/flavor is increasing over time after having kegged the beer. From everything that I know acetaldehyde is generated from a few different ways, active fermentation, and yeast death.
In my mind, since the flavor is developing after the active fermentation it sounds like everything you are doing before transferring to keg is going well. If you had the apple smell taste in the early stages then most, along with me, would guess its generated from the active fermentation. Lets rule that out though.
So the next main thing to look at would be yeast death in my opinion. I have read through this thread and saw a few things that look like a potential possibility. One is initial aeration of the wort. I don't remember that being addressed. The yeasts health will be directly effected by this. Second is nutrients, which has been addressed. Last, which I don't remember anyone asking. Do you cold crash before kegging?
Last, but maybe the most likely suspect in my opinion really is sanitation. Aceto bacteria can and will cause a cidery taste, and once its there there is nothing that can be done about the taste. In my opinion since its a taste that is getting stronger over time after the primary fermentation, this would be your most likely culprit. What you are using to transfer from primary vessel to keg, or even the keg itself may have some little jerks lingering around waiting to ruin a good batch of beer.