Like I said, I'll report my findings when I get around to sanding the hull. Regardless of how difficult it turns out to be in practice, I will not sand through the glass at the transition. Luckily, it's not a matter of guesswork, it's a matter of being careful. I may find out that it's more work than it's worth and I'll go back to placing the partial layer on top. Alternatively, I might find that it's not so difficult to sand at the transition. In which case, I've saved myself some aggravation during wet-out.

I surprised that you've written-off this technique completely, having never tried it yourself and without any empirical or anecdotal evidence that it's responsible for failures.

Upon further refection, I'm sure a case could be made for sand-through to be more likely with the partial layer on top. With that arrangement, you have to concentrate the sanding in that area to do the feathering. It's very likely that you'll hit some of the full layer of glass. Sounds like either method requires care not to damage the full layer of glass.

Ross