The potential problem due to not prepping the surface may not show by a simple break test. There is some chance of delamination between the layer down the line.

To test for delamination, you would want to do some sort of pull test. Here is a fancy example: https://youtu.be/S3so3ktUA8E

A test I have heard of for varnish is to scratch an X on the surface with a utility knife to cut through the layer, then stick good sticky tape to it and pull up the tape. Inspect the X to for signs of delamination.

Frankly, your worst case layup would probably work fine for most of our kayaks. There is some reduction of strength because of non-optimum prep work, but it is probably still strong enough for the forces we would generally subject our kayaks to.

That said, years ago I did get some delamination of fill coats due to poor prep work. It took a few years for the problems to show.