No, you don't want to use the chemical additives. Do you know the KH? At a guess, the high pH is being caused by the water softener (it works by replacing minerals in the water, doesn't actually remove anything from the water). Is there a water tap before the softener you could use? What type of water softener is it?
For what it's worth, with UGF, the bacteria are all in the gravel. Covering the olf gravel up, so long as you don't make the substrate deep enough to completely block the water flow, is not a problem at all. The UGF works by pulling or pushing water through the gravel bed, supplying the bacteria all through the bed with food and oxygen.