Hmm, interesting questions. For visibility I think you could do this relatively easily. You could have your corner actors (that block RangeTrace) each have a faction variable. On a new faction’s turn you would loop through all of these actors and enable/disable collision based on the faction of the corner blockers relative to the active unit’s faction.
Pathfinding is more difficult. I have one idea which should work, though there might be a more elegant solution I’m not seeing. You could create a new TMap for BP_GridManager to hold your special corner blockers. You would add them to a grid index in this TMap once spawned and remove them when they are destroyed. They are at a corner, but you would store them in one of the surrounding tiles in a consistent manner (say, always the tile to the north west of the block’s location). Then you could create a new custom pathfinding type. You could use FindAndAddTilesNoDiagonals as a starting point. On the CheckIfStraightAdjacent branch in this function, false means a diagonal connection. Here you could check if the connection is passing a blocked corner by checking your custom TMap and, if the index is occupied, check the faction of the corner box against the faction of the active unit, continuing pathfinding if they are the same.
That explanation might have been a bit confusing. Let me know if this is not enough to go on and I will provide a more detailed explanation.