An important part of deniable encryption is that you can "provably" deny data isn't there when it is. You want the authorities to agree that when you unlock your volume they see "all" of your data so they don't hold you in contempt. If the total changes is less than the data visible, the assumption is you're hiding more data. Adding noise isn't deniable. In fact, it would hurt you: If you can't prove your noise is noise then the authorities are going to assume you are hiding data.

I think you make a good point: the only way to be secure in this method is to A) work on an uncompromised machine and B) write each layer in series and C) never change it again.