Yes, I am aware I didn’t mention the castle. That is because, unless the castle is shown in the show that it has relation to the chest, I am not counting it as such. If, however, there is a connection, I’ll include it in the final imaged tally and in an unofficial counting in this description.
The Y/M sign relates to the shiny objects that the ponies get, which are implied to have some connection to the chest.
Tallying up, the ratio of plot:filler episodes is 7:12 (not counting the castle), 9:10 (counting the castle).
Hm… Come to think of it, including the castle might help make it more of a plot-based season.