Head gasket leaks I've seen. Have been along length of head. We've heard, but I've doubted, head gasket leaks without overheating warping head. When head gets warped, this would almost certainly be along length. These leaks are almost always visible, without removing any more than under shield and engine cover. So the shops you've had looking, should have spotted. They also are under intake manifold or sides of block. Not in front of engine.
But let's assume a fluke, and head gasket leaking at front end. If head gasket at front BK2, it be easy to confirm. First by looking here (pink circle). Can be seen easily with air pipe off and a flash light. Also from below with #1 under shield (Skid #1) off.
Second, by removing BK2 upper plastic timing cover. Red arrow is pointing to head gasket. I would want leak confirmed, before removing a head.
Again using a borescope, they can pin point and provide pictures of point of leak.
Front AND Rear Water bypass, attaches to heads at yellow circles. Water pump port for water cross over pipe goes into back of water pump (SM O-ring sealed) blue circle.
If at BK2 front Water bypass gasket (yellow), It would be seen on top front of head. SM O-ring would leak into engine valley. They "may" also flow down front of engine BK2 side.
The most difficult would be the water pump leak, but can be done with bore scope.
Water pump leaks, not at water inlet FIPG seal (rectangle port water inlet attaches to) or small O-ring port of water cross over pipe on back side. But actually from a water pump failure. Are most often found at of two points. First being on LH side (BK1), where there is a weep hole in water pump. Which should first show-up on BK1, just below upper timing cover. Than weep at lower timing cover. This weep hole comes from shaft of water pump. When water pumps fail, bushing in shaft goes bad, and leaks at weep hole first. Then once really bad, at drive shaft of water pump leaks.
WP weep holes (on BK1) often, have some sign of weepage as seen here.