Remove genetic things from lings entirely.

[BlackPhoenix]

So you're mixing lore reasons and mechanics reasons a lot here.

Lorewise: Changelings are adept at mimicing human physiology, why couldn't their genes be similar? Hell, the two things they can look like are humanoids and monkeys. The Shambling Abomination is just a whole bunch of humanoids and meat thrown in a creepy pile.

Mechanics: Changelings with DNA powers aren't overpowered. They just aren't. They have no way of speeding the process up compared to anyone else getting the powers.