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Retouching gradients as invisibly as possible is always difficult, especially with high contrast and accentuated film grain.
To simplify the retouching process, I would try working with a standard image that has lower contrast. Reproducing HP5+ film doesn’t make things any easier.
Then, I would export the result as a 16-bit TIFF to obtain a standard positive *) and then apply film characteristics and / or grain, contrast, ClearViewPlus, etc. – This should help you achieve better retouching.