One final note: apparently one needs to keep things "low-level" with CellGroupData[], but also, for the groupings to work, it needs to be fed back into NotebookWrite[] and not CellPrint[].

Hello. Here is the link to the SE discussion. (Instead of continuing both here and there, is there a preferred place?)

Certainly my intentions were to keep my question as simple as possible, and not to post an XY problem.

Based on Gianluca's explanation, I moved to string/text, rather than expression, i.e.:

TextCell[StringForm["`` `` ``", "c", Plot[Sin[x], {x, -2, 2}], "a"]]

This does the job.

However, since this was feeding back into CellGroup[], and from Kuba's answer (here and at SE), I need to move to the lower-level Cell[] and CellGroupData[], I can no longer use TextCell[]. Sigh.

So I am back to boxes (as Kuba suggested in SE), with:

Cell@CellGroupData[{Cell@
    BoxData@RowBox[{"c", ToBoxes[Plot[Sin[x], {x, -2, 2}]], "a"}]
   }]

which can be, e.g., CellPrint[]ed.

Gianluca, thank you for your reply, as well as the explanation of the "interpretation" context. (I still do not understand where the "zztop" came from.)

Do you have a suggestion for a workaround?

Again, I am trying to concatenate together strings, and expressions like plots, and to retain the input order. I am using ToExpression[] since this is all fed back into CellGroup[ExpressionCell[ ... ]].

Thank you.