G'day Alkay, bit late on the scene as been busy doing plenty of work and builds and just catching up on some threads and found this one.
Would not recommend deep mahogany for the spalted top as you may as well go a solid colour and kiss goodbye any grain definition....the colour will most likely come out very, very dark brown.....almost Cadbury Milk Chocolate colour if not too careful as the black grain popping will only add to the overall darker theme.
If you have not already started staining maybe continue with a few sheets of wet kitchen paper towel to help draw out more of the dark grain popping efforts. Tend to think your original plan of Cedar would look better as it is a lighter colour shade that would allow the grain popping to show up.