A header is above the window, this stuff is below the window, so it is just filler. This window has been made bigger at some time you need to check the header above it. Above the window there should be a 2 ply header, today would be 2x10 supported each side with a jack stud and a king stud beside that. So when you have it open like you have you should see 2 studs on each side of the window with solid blocking right to the foundation. Your contractor didn't help this but the problem was already there. If the upstairs is platform framed as in the upstairs walls are built ontop of the floor then you can just build the temp wall, jack it up and reframe the window and wall. You should have your engineer come back and look at it and check you loads incase it needs more than a normal header.