Ok I’ve nailed it down. To update prices ONLY where you have parent and child rows where the parent row has all the product information and Option1 Name is Variant, Option1 Value is X and price is Y and the child row has no title or Option1 Name (inherited from parent row) the only columns we MUST have in the import to update prices are
Handle, Title, Option1 Value and Price.
I have tested this extensively. You can’t do it in less rows which makes sense when you think about it. This is the minimum information shopify would need to be able to differentiate between different items and update things.