Owen, your original surmise about the Sherbourne is correct. As far as wooden kits are concerned, you can't beat Caldercraft/Jotika for quality of materials and documentation. Billings, on the other hand, is problematical. Having said that, you have risen to the task admirably.
For your other issue, plank vs solid hull, I for one wouldn't waste my time on the latter. If you can nicely plank a hull (you have!) you can build "anything". I would say only that plank on frame gives no advantage over plank on bulkhead unless you want to expose to interior.
Congratulations on your work, very nice indeed.