If you have to check everything, then there are no "key checkpoints."

In fact, the key checkpoints will vary with the product, the process, and what's already known. For example, if the new product is substantially similar to an existing or former product, the focus in the trial production run might be very limited. It also depends on what the customer expects; if PPAP is involved, a complete layout might be required regardless of the pedigree of the new product.