I love the idea but I do notice one problem right off the bat similar to the torch issue in Book I.
In that if you have quick enough hands you could potentially attack then switch to parry immediately after attacking and thus have an immediate defense bonus. (Assuming the switch is similar to lighting/dousing a torch)
Admittedly this is a whole lot more reasonable than lighting and dousing a torch multiple times per round, and is even possible in the rhythm of a battle in reality so it's a potential exploitability but it is a far more reasonable one. However it would remove some of the strategy and I doubt that this would be the intended situation.
Of course you may very well have designed it that during the enemy's turn you can't change your current combat mode which sounds more like you described.
Potential exploitation issues aside, I love this idea, especially the parry and finesse modes as I got tired of being hit too often and being unable to land decent hits back. Any modifications to the game that either require or encourage more strategy are good decisions in my book. One of the things I loved about Book I was using the environment to my advantage, i.e. powder kegs, portcullises, funneling my opponents, trapping my opponents (via demon oil), etc.
Parry mode could potentially greatly assist in this as well.
Additional skills could be used to assist these modes or even perhaps allow use of them as in cartography was required for the mini map. But even just defaultly placed I love it.
Please excuse my made up words,
