Late reply, but a good question! Even away from the center there is no gravitational field inside a spherical shell. If the particle is not in the center, but somewhere else, let’s say somewhere to the right of the center. Then you are correct that the force from any small bit of shell on the right side (closer the the particle) pulling the particle to the right is larger than the force from the left side (far away from the particle) pulling the particle back to the center. However because the particle is not in the center, a larger part of the sphere is on the left of the particle pulling the particle back to the center, than on the right of the particle pulling it further away from the center. So, while the force by any small bit of area from the closer side to the sphere is larger, there are more bits of area on the far side of the sphere. These effects cancel out exactly so the net force is 0 anywhere inside the shell.

To your second question, a different way to look at it gravitational energy being negative is through kinetic energy. Take two particles which are not moving so they have no kinetic energy. The gravitational attraction will cause them to move towards each other, so they build up velocity and gain kinetic energy. This kinetic energy has to come from somewhere: the gravitational field. So to conserve total energy, if their kinetic energy increases, the energy of the gravitational field must decrease. The same happens if the sphere collapses without generating power. The energy adding an extra region of gravitational field as mentioned in the talk goes into the motion of the shell. This motion can be used to generate power (only as thought experiment of course)