Julie,not being familiar with this class of sailboat,is there another layer of planking going over the diagonal one? By the way,the word is "reversible" amongst the museum weenies ( The conservation techs).
Vinegar is good for getting hide glue loose. But,it sounds like you need to resort to a chisel to remove the offending planks. But,remember vinegar , if you ever have to loosen an acoustic guitar neck. Vinegar and steam. You have to drill a small hole down each side of the neck dovetail, and insert a steam pipe made of model airplane brass tubing. Inject vinegar first and re supply it for a few days. Then steam. There are different ways to remove necks. I made my steam "boiler" from a 1 gallon size powder holder from a tank gun (as in war!). Apparently some do not use FIXED shells (those with brass casings), and the powder is loaded separately. I'm not sure how that works. I had thought that only larger caliber cannon aboard ships used separate powder in bags. Anyway, I sit it on a hot plate. It has a thick,black rubber hose with a brass nozzle I turned on a lathe. I only had one powder can,though. They are VERY heavy duty!! And GREAT for storing things.