Kolo! Greek alert!
In Attic Greek (which is where my textbook is) there is a kind of "on the other hand" sense "δέ", but I have had a tough time (the last time this came up, some months ago) running down any sense of certainty about the particle.
Maybe Kolo or some other Greek scholar can give us some words of wisdom.
You are absolutely right, MD. Indeed "δέ" can mean "on the other hand" in modern Greek too, but it also clearly can mean "but", though at least in modern Greek we generally use another word. In Luke 11:28 (not 26) the "δέ" most likely translates as "but" as would the same word in Luke 8:21. The "δέ" in Matthew is usually translated "yet" or "on the other hand". But bd, there is no intimation in the Greek that those who hear and keep the word of God are more blessed than Panagia.