Seem to recall once reading, the casualties from Slapton Sands, were conveniently tagged onto those from Omaha beach.

I think you're right - it may be that the same unit was at Slapton Sands and Omaha, which might have been one reason why it was done.
I think it was also political (aside from the careers of the officers involved) as the 'european war' wasn't popular in America and, if the public had found out they'd killed their own troops, Roosevelt would've been under far more pressure to pull the plug and concentrate just on defeating Japan.
As an aside, because some (about 10 I think) of the engineers knew the details of D-day, it was nearly cancelled until they were able to identify all of their bodies (the E-boats had been seen going through the wreckage of the ships).