I have changed it so that the template that figures out the width uses a
param i pass it, but how can I figure out if the table is inside a table
cell, and pass it the value of the parent table cells parent fo:table-column
width?

Use something like
 <xsl:if test="ancestor::table">
Better yet, always pass the current block width down through all
templates matching block level elements, and pass the page width
in the top level template.

<xsl:template match="table|tbody">
  <xsl:param name="blockwidth"/>
  <fo:table>
  ...
   <xsl:call-template name="process-col-width">
     ...
     <xsl:with-param name="parentblockwidth"
          select="$blockwidth"/>
    ...
  <fo:table-body>
    <xsl:apply-templates>
     <xsl:with-param name="blockwidth"
          select="$blockwidth"/>
    </xsl:apply-templates>
  </fo:table-body>