Jings, Crivvens and help ma Boab! That was fun and very clever in what changed - and what did not - in the end game. I confess to being a dedicated fan of the subject, so may be prejudiced.
slithytove, chess notation for knight inserted in the middle of a four letter word meaning place of authority, and if you drop the A you get base, as in logarithms, to give the final letter.
Thanks, Quisling. That about completes the puzzle for me, and fits with my thoughts on 26d as far as the def. It's the parsing that I can't get. I figure PA for per annum, but the rest eludes me.