Remoak
Runway visibility is measured adjacent to the touchdown areas and at the centre of the runway at a height of 5 metres. This is intended to be indicative of the pilots view when landing. The automated service at Richmond reported a visibility of 200m at this 5m height. Besides, OAT 4 degC, dewpoint 4 degC and calm wind screams fog.
In what fairy tale land does that translate to isolated patches of 6 foot thick light mist?
The air traffic controllers statement is ambiguous and in conflict with the AWIS. Although if he was in the tower its possible that he was near the top of the fog layer - in which case the sky would look blue to him. The tower at YRSI looks like its about 15m tall based on scaling its shadow in Google Earth.
From the pilot's point of view the thickness is irrelevant if he couldn't see the ground and the reported weather was well below ILS minima, then Richmond airport didn't exist for his purposes.
Based on the graphs I made of altitude, airspeed & vertical speed, My speculation is that the pilot was not aware of the second problem until abeam or past Richmond. I think that there are signs he was unaware of an issue with the LH engine at the point he turned around.