In music, chromatic mediants are "altered mediant and submediant chords."A chromatic mediant relationship defined conservatively is a relationship between two sections and/or chords whose roots are related by a major third or minor third, and contain one common tone (thereby sharing the same quality, i.e. major or minor).
If you ...
Now, I know that the second to last chord, I want to be either a B7 or a D#°7, since both of those are dominant function chords that will smoothly resolve to E major.
I agree with you that the omnibus progression has to be in B7.
But can E major be reached by an omnibus progression from C major or a chord closely related to C major?
I don't know a trick ...