Can a light glider without thermal protection land from the orbit, starting from the orbital speed?
No.

The stagnation point temperature goes up with the square of true air speed.

Temperature dissipation is proportional to true air speed and density.

Lift is proportional to the square of airspeed and density.

The lower wing loading of a glider (compared to the Shuttle, for example) means that all of the reentry will be slower, but that is not necessarily an advantage. Note that the Shuttle needed external cooling to prevent heat stored in the heat shield to dissipate into the Shuttle's structure. If the glider flies a longer reentry, it will be closer to thermal equilibrium. Also, the aerodynamic shape of a glider is a disadvantage here because the lowest heat load is possible with a blunt object - this is why reentry vehicles look like they do.

Just to give you an idea what temperatures are involved:

If we express speed as Mach number, a typical reentry speed would be Mach 25. This makes the reentry temperature at the stagnation point $$T_s = T_{\infty}\cdot \left(1 + \frac{\gamma-1}{2}\cdot Ma^2 \right) = 126\cdot T_{\infty}$$
at Mach 25. If we assume 195K at the edge of space, this comes out to 24,570K - theoretically, because ionization effects will cause the eventual temperature to be lower and the ratio of specific heats $\gamma = c_p/c_v$ is no longer constant. To calculate the real value needs a non-equilibrium gas model since the nose radius of typical glider wings and tail surfaces is in the order of one centimeter. The distance between the detached shock wave and the structure is too small for the gas molecules to reach equilibrium.

On the other hand, the epoxy resins used in glider construction cure at room temperature first and are then tempered at about 60°C. The glass transition temperature $\text{T}_g$ of such resins is at best a couple of degrees above the curing temperature. $\text{T}_g$ marks the onset of weakening of the composite matrix, and heating the glider above it will permanently damage it. Given that the glider will stay for dozens of minutes in air of a temperature of several thousand degrees and has no thermal protection will make sure that all which reaches the ground is a charred lump of material.