I have inserted a few statements for debugging like
v1: { v1 }
I would like to make them invisible rather than delete
them, because I might need them in the future if I make
changes to the mail merge document and new bugs appear.

No problem for making the "v1: " part and the paragraph
sign at the end invisible. However, it seems to be
impossible to make the { v1 } invisible, too. It appears
with the thin dotted line alright, it's invisible as long
as field codes are on (and invisible symbols are switched
off, of course), but as soon as I press Alt+F9, the field
result of { v1 } is showing.

Is there a way to make that invisible?

TIA,
Monika.