I've written an intermediary stylesheet to split out <list> elements containing <pb> and <fw> children into separate lists with the <pb>s and <fw>s outside them. This is processed prior to the main transformation, and has solved the problem for lists. I may revisit it for lg/l at some point. For the record, here's the template (the only non-pass-through template in an identity transform):

<!-- Split out interrupted lists into a sequence of lists with the interruptions

      between them. -->

  <xsl:template match="list[child::fw or child::pb]">

<!-- Stash a reference to the current list so we can easily refer to it in later nested code. -->

    <xsl:variable name="thisList" select="."/>

    <xsl:for-each-group select="child::item" group-adjacent="count(preceding-sibling::fw) + count(preceding-sibling::pb)">

        <list>

          <xsl:for-each select="$thisList/@*">

            <xsl:copy/>

          </xsl:for-each><xsl:text>

</xsl:text>

          <xsl:for-each select="current-group()">

            <xsl:copy-of select="."/><xsl:text>

</xsl:text>

          </xsl:for-each>

        </list><xsl:text>

</xsl:text>

      <xsl:variable name="lastItem" select="current-group()[position() = last()]"/>

          <xsl:for-each select="$lastItem/following-sibling::fw[preceding-sibling::item[1] = $lastItem] | $lastItem/following-sibling::pb[preceding-sibling::item[1] = $lastItem]">

            <xsl:copy-of select="."/><xsl:text>

</xsl:text>

          </xsl:for-each><xsl:text>

</xsl:text>

      </xsl:for-each-group>     

      

    

  </xsl:template>