"Give it away, give it away, give it away now! Give it away, give it away, give it away now! Give it away, give it away, give it away now! Oh, oh yeah!" I couldn't help but think of the Red Hot Chili Peppers big 1991 hit "Give It Away" while looking at this scene which has someone trying to keep a secret! Enough said!
There are no clues in the older series. Mr. Plotter is a brand new concept. I'll give you this much though: his identity, his motive, and the execution of his own master plan are sprinkled throughout this series and by the end of this chapter it will all make sense.
I went through all the new comics, including whatever ones I missed. You said his motive is sprinkled throughout this series as well. At this point I'm thinking Tom rather than Mr. Seymour, because he doesn't like anyone looking at his sister. Also because at the end of Pigwarts, you mention that he is the least of your favorite characters to write for and he will probably make another return. Hmmmmm..... Tom does seem to fit a motive that would make sense.