I suspect that the best answers that you're going to get are "it varies" and "it depends" for the first question, and "yes, it is" for the second.
Someone with a lot more aerodynamics gravitas will correct my details, but here's what I know:
The boundary layer at the leading edge is going to be very thin and laminar. At some point past that it will separate -- this point of separation will be earlier in the airflow on the lifting side, and later on the non-lifting side.
Exactly where it separates, and how thick it is is going to depend a lot on the details of the airfoil, the angle of attack, the flap deflection, and the surface finish -- a nice smooth surface, and a nice smooth airfoil are going to make it separate later. A rough surface at the right place (See PJ Rowland's turbulators) will make it separate sooner but will keep it thinner back to the back of the wing. In level flight it will show a lot less turbulence, much farther back on the airfoil, than in a square corner. Taping the hinge lines will (I think) keep it thin at the flap leading edges, and will (I know) make it more consistent from wing to wing, flight to flight, day to day, etc.
Someone really needs to build a wind tunnel that operates at 50 MPH and will hold a 12-inch chord wing, and then take some pictures of smoke trails at various angles of attack and flap deflections.