Re: number collapsing

One observation:

       <xsl:when test="$begin castable as xs:integer">
         <xsl:variable name="begin2" select="$begin" as="xs:integer"/>

This may fail if $begin is a string. The string "123" is castable to
xs:integer, but not assignable to xs:integer.

Better to write the second
line as

<xsl:variable name="begin" select="xs:integer($begin)" as="xs:integer"/>

to force the cast.

I think your new problem can be tackled using a "group-starting-with". The
only problem is that this requires nodes rather than atomic values. So we'll
start by putting the numbers into a temporary tree:

<xsl:function name="f:ranges" as="xs:string">
  <xsl:param name="pagelist" as="xs:integer*"/>