I am guessing it is the case.considered you wish to add the sorting to. Currently, you are doing this
<xsl:apply-templates select="//case.considered" mode="x"/>
To do sorting, you would need to change it to this
<xsl:apply-templates select="//case.considered" mode="x">
<xsl:sort select="case.ref/citetitle" />
</xsl:apply-templates>
This does assume only one case.ref element per case.considered element though. If there are more than one, only the first one is used in the sorting.