Assuming we are talking about modern digital aids (analog aids acted much like you suggest) then they will (should) be set up to amplify less when the incoming sound is louder. They also have a cut off threshold above which they will not go, your aid can't get to 142 dba, it can't.

Also, and this is where you need to talk to a knowledgeable Audiologist (I went through 6 trying to find the one I have now), the whole thing is a lot more complicated then just trying to add a reverse curve. Once sound reaches a threshold where we can hear it then, in general we hear as well (some suggest better then) normal. We may not understand, but that is due to issues other then volume. So in your example, with a 70 db loss and a 75 db conversation should be hearable, the loss is not a subtraction but a threshold. Now you may not be able to understand it but that is more likely a signal to noise issue, which is best solved with an assisted listening device.