First of all, it is correct that the universal gravitation of a hollow spherical shell to a particle inside it is 0, and it holds true at any point inside, which can be proved by mechanical method or flux theorem.

Then the change of field strength is explained. The field strength can be expressed as a negative gradient of gravity potential (gravity potential U=-GM/r). Considering that the whole field distribution is spherically symmetric, the gravitational field strength e =-du/dr. Because the gravitational potential inside the spherical shell is constant and the outside is hyperbolic, the point on the spherical shell is non-conductive, and the gravitational field strength at this point is not defined mathematically and does not exist physically. Just like your calculation "gravity is either 0 or GMm/R*2", there is no error, the field strength at this point does not exist, is the change of gravity continuous?

From a real point of view, any point in space should have a gravitational value (regardless of some corrections and special circumstances under the condition of general relativity), so similarly, the spherical shell model is also a mathematical abstraction, that is, there is no object with zero thickness and mass in the world, as long as there is a thickness field strength, it is continuous.