The glide angle is between the horizontal and the flight path. For maximum glide range you require minimum glide angle. If you could achieve a gllide angle of zero you would never reach the ground, so your glide range would be be infinite. But in the real world your glide angle will usually be more than zero. so your glide range will be limited.
If you do the maths you will find that glide range (in still air) from any given height is equal to the lift to drag ratio multiplied by the height at which the glide starts. So if you fly at the speed at which lift to drag ratio is greatest, you will get best glide range. This will also mean that you have the minimum achievable glide angle.
Best lift to drag ratio occurs at VMD, so you get best glide range by flying at VMD. If you change the weight or configuration, you will of course change your VMD. But provided you fly at the VMD for that weight and configfuration, you will get your best glide range and minimum glide angle.
The weight of an aircraft does not affect its lift to drag ratio at VMD, so glide range is not affected by changes in weight. Increasing weight for example, simply means that provided you fly at VMD, you will fly down the same slope faster. You hit the same spot on the ground but get there sooner.
If your sink rate is zero you will never reach the ground, so your glide endurance will be infinite. This is possible in updrafts, but you will never achieve zero sink indeffinitely. so your glide endurance will be limited. But you will always get best glide endurance if you minimize your sink rate.
To get maximum glide endurance you must fly at VMP. To understand this we must consider the energy situation. Flying through the air requires energy, which is usually provided by the engines. So when the engines fail we can get no more energy. Whatever energy we have stored in the aircarft is then dissipated in the remaining gliding flight.
At the time of engine failure the total energy of the aircraft is the sum of its kinetic energy (1/2mv squared) plus its potential energy (wieght x height). After it has landed and come to rest its energy is zero. So to maximise glide endurance, we must minimise the rate at which we dissipate the energy that is stored in the aircraft.
The power required is a measure of the rate of energy dissipation, so to mimise the rate of energy dissipation we must minimise the power required. This is achieved by flying at VMP. But changes in weight or configuration change the power required, so varying aircraft weight or configuration will change the glide endurance.