OK, a Slide-In Truck Camper. I love those for Canada and Alaska! Same cab-over as a Class C. I know this is frustrating, and you'll lose a month of RVing fixing it, but so far it doesn't look all that bad. At least from the "We've Seen Worse" standpoint.
Is the floor (mattress surface) of the cab-over compromised? How does the other front corner look?
I think you'd best figure out how to get that flat wood-grained piece out, even if you have to make another one. My "Why" for that, is to be able to get a good surgical field in the lower front corner.
I pray I never have to do this repair, but if I did, thinking this: Make up framing to form essentially a "C" shape that leads forward below the roof, follows the turn to the front slope, makes the sharp bends at the lower front, then continues back, along the cab-over floor. Maybe cut it out of 1" marine plywood, leaving a hole for the window.
Then strip out the interior paneling and insulation. Glue "furring strips" to the inside of the coach's outer skin to attach paneling to, then re-assemble the outside. After that, insulate with foam insulation board, re-panel, and install the window.
While you're working, pull the Clearance Lights and make sure they aren't contributing to the leaks.