Mira,
Well, “don’t set up wards that will stop a fleeing traitor” is an awfully specific and overt manipulation to work on older minds (remember they made a big deal about that). Peabody’s ink only worked because it was very subtle manipulation.
Something about Eb makes my spidey senses tingle.
The ink worked on all minds, maybe not to the degree as on younger minds, but enough so all the decisions made by the Senior Council in the has dozen years or so had to be revisited. Senior Council minds are not younger minds. So I think yes, Eb was affected, I don't discount your spidey sense, I'd also toss in beginning stages of dementia as well.
Peabody was able to cause so much havoc because he was totally trusted, as secretary, all decision papers and orders went through him, these all had to be signed off by a senior wizard.. That made Peabody the perfect plant, the ink affected everyone without them realizing it.