I don't know if this explains your problem but that code initializes the 'service-url' variable with a Url containing a query but opens the stream using a different version of the Url that replaces the original query with "?format=xml&rows=5000". In other words, the set-query method on Url replaces the entire query with a new one; it does not append to the existing query. The Url class does not provide any API for merging queries, so you would have to write something like this to add to a possibly non-empty query:

{define-proc {append-to-url-query u:Url, s:String}:Url
    def old-query = u.query
    def query =
        {if old-query == "" then
            {format "?%s", s}
         else
            {format "%s&%s", old-query, s}
        }
    {return {u.set-query query}}
}