I have tried to use the recommendation raised previously in this forum
to produce a separated by commas list, but I do not obtain the awaited
results.

If you don't like the dot after each semicolon, just move the
generating element after the for-each, from here
   >               <xsl:if test="not(position() = last())">
   >                 <xsl:text>; </xsl:text>
   >               </xsl:if>

<xsl:text>. </xsl:text>

   >            </xsl:for-each>
to
   >               <xsl:if test="not(position() = last())">
   >                 <xsl:text>; </xsl:text>
   >               </xsl:if>
   >            </xsl:for-each>
>>>>               <xsl:text>. </xsl:text>

It's probably also a good idea to wrap the text "Asistieron:"
in an xsl:text element.