OTOH, I am also aware of the minimum sink speed for a glider, which is at the maximum lift speed ( just above stall), and lower than the max endurance speed for a powered aircraft.
I don't think thats correct.
Obviously with a glider you remove any powerplant issues regarding thrust or power output versus speed, and it purely becomes a lift and drag versus airspeed problem.
I agree that the minimum sink speed is very low, but I don't think that is the point of maximum lift.
Given that Lift = W Cos (Angle of Descent) you will be at the point of maximum lift when you minimise the angle of descent, i.e. at best L/D ratio.
Where as the minimum sink speed is the point of minimum power required. as mentioned above, the point of minimum (Drag x TAS).
So in effect, by waffling along at low speed you put up with more drag (being below Vmd) for the bonus of a proportionally lower TAS. The downside is that this is not the optimum L/D ratio and therefore your descent angle is actually steeper, and hence lift is smaller.
So, in summary, minimum sink speed is not equal to maximum lift.
On the other hand, its been a while since I taught this so I could be talking out of my fundamental orifice. It wouldn't be the first time!
BTW since power=drag x TAS, and given that at any particular altitude TAS is proportional to RAS, you can find the minimum power required speed from a drag graph. Graphically, drag x TAS is proportional to the area of a rectangle formed by drawing lines from your chosen point on the graph to both axes. It is reasonably obvious (pictorially) that this point is way below Vmd.
CPB