Hi Pete, It's wonderful the many forms of wood boring bugs there are, unless we can identify the type, we can't be sure which we are dealing with, small dusty holes, if when found, could be active woodworm or exit flight holes, you never know for sure which or what has happened..
A house we purchased once the Building Surveyor missed tell tale holes round the loft hatch, it was that close under his nose .. I had no come back over what had been shoddy full survey... woodworm .. it happens.
On balance would it be unstable in extending the form out along the ledge as it is or make the ledge longer, to extend the rear part an add a stone as you've done with your pieces.
Cheers David