Why does the \marginpar not appear on the left page?

You have adjusted your page margins, but haven't adjusted the marginpar dimensions to match. I suggest adding parameters to your call of the geometry package:

\usepackage[
  margin=9mm,
  marginparwidth=7mm,     % + <- Width of your marginpar
  marginparsep=1mm,       % + <- Gap between text block and marginpar
  paperwidth=107.95mm,
  paperheight=174.63mm,
  bottom=18mm,
  ]{geometry}

This will also allow you, if you need, to see the content without re-specifying alignments.

Use this:

\DeclareRobustCommand{\mymarginpar}[1]{%
 \marginpar[\raggedleft#1]{\raggedright#1}}

and use \mymarginpar{$\psi$}

The problem is that the psi is off the page in your case.