That'd have me scratching my head as well at first (
), but logically........
1) IF the heater flow and return pipes behind the glovebox are both truly hot (once car has warmed up), then there must be hot water flowing around the matrix.
Its important that you ARE checking the pipes under the dash because the piping that emerges near to the cylinder head can get hot through conduction only and mislead you into thinking there is a flow.
2) You're getting a jet of hot water out of the bleed valve which also tends to indicate that there is water flow ....
3) Assuming the blower motor IS actually blowing air out of the ducts, (is it?), then the only reason I can think of (short of a blocked main air pipe / blower motor) for still getting cold air is that the 'hot / cold' duct flap is stuck in the cold position making the blower push cold external air through the car....(i.e. completely by-passing the heater matrix).
What does the hot/cold knob feel like?....gone very stiff? or suddenly slack ? .....i.e maybe the operating cable has seized / broken?
I think I'd go back to the glovebox out stage and with the aid of a torch see if I could verify that the hot/cold flap IS actually moving to and fro.
If its none of these then I agree - its very odd indeed!
let us know how you get on......I'm fascinated now!