When only some neurons drive a cursor, does the brain find and change those ones? · Grey Matter

Partly.

When only some neurons drive a cursor, does the brain find and change those ones?

Partly. In 2008 monkeys moved a cursor in three dimensions with a few dozen motor cortex neurons, and the experimenters then rotated the mapping for a subset of them, so those cells now pushed the cursor the wrong way. The animals adapted, and the changes in firing reflected two things at once: a general shift of strategy across the whole population, and adjustments weighted by how much each neuron contributed to the error. The brain can single out the cells that matter, at least in part, which is exactly the credit assignment problem solved on a small scale.