When I need to model a corner like that with no curve, I will put a short alignment segment of a few hundredths at 45 degrees at the corner. Then create a region for that segment with just one assembly. You can either set a target to get it to stretch into the corner or create a new assembly that's 1.44 times longer. I think I would stop the typical assembly where the curb radius starts. Through the corner from there, I would use an assembly for the outside lane only. To model the inside in that area, create an assembly with the lane on one side of the marker and the curb on the other that follows the EP and targets the main alignment. Make sure you add an assembly that hits the corner or you'll end up with a small hole there.
Hard to explain. Hope it makes sense.