Use Ontology to correctly assign data types in Virtuoso

I have ingested the Geonames RDF dump (https://download.geonames.org/all-geonames-rdf.zip) into a Virtuoso instance, and I've been running queries against it with varying degrees of success. However, I've found that certain objects have the incorrect datatype. For example, population is encoded using xsd:string, and therefore trying to sort by population ends up sorting the results in lexicographic order:

PREFIX  xsd:  <http://www.w3.org/2001/XMLSchema#>
PREFIX  gn:   <http://www.geonames.org/ontology#>

SELECT ?country ?name ?population (datatype(?population) AS ?type)
WHERE {
    ?country a gn:Feature .
    ?country gn:name ?name .

    # A.PCLI is feature  code for 'independent political entity'
    ?country gn:featureCode <https://www.geonames.org/ontology#A.PCLI> .
    ?country gn:population ?population .
}
ORDER BY DESC(?population)
LIMIT 10

country	name	population	type
https://ift.tt/3Fg8k6V	China	1330044000	https://ift.tt/2VRgVvo
https://ift.tt/3c4uLiM	India	1173108018	https://ift.tt/2VRgVvo
https://ift.tt/3HhNLc1	United States	310232863	https://ift.tt/2VRgVvo
https://ift.tt/3oj64ou	Indonesia	242968342	https://ift.tt/2VRgVvo
https://ift.tt/31Ylaby	Brazil	201103330	https://ift.tt/2VRgVvo

I know I can cast the variable to get the correct result like so ORDER BY DESC(xsd:integer(?population)), but once my queries get more complicated, this no longer works. Specifically, when running sub queries and using the results to apply further logic. For example:

PREFIX  xsd:  <http://www.w3.org/2001/XMLSchema#>
PREFIX  gn:   <http://www.geonames.org/ontology#>

SELECT ?cityName ?countryName ?population datatype(?population)
WHERE
{
    ?city  gn:parentCountry ?country ;
           gn:population    ?population ;
           gn:name          ?cityName .

    ?country gn:name ?countryName .

    {
        # a) SELECT ?country (MAX(?population) AS ?population)
        # b) SELECT ?country (MAX(xsd:integer(?population)) AS ?population)     
        # c) SELECT ?country (xsd:string(MAX(xsd:integer(?population))) AS ?population)
        WHERE 
        {
            ?city   a                gn:Feature ;
                    gn:featureClass  <https://www.geonames.org/ontology#P> ;
                    gn:population    ?population ;
                    gn:parentCountry ?country .

        }
        GROUP BY ?country
        ORDER BY DESC(?population)
    }
}

Select a returns the populations in lexicographic order, as before.
Select b correctly orders the populations, but seeing as the result set has cast the population to integers, I can no longer match the city using population outside the sub query as I'm comparing strings with integers. So b returns an empty result set.
Select c was my attempt at recasting the results back to strings in order to be able to match them outside the sub query, but this ends in a timeout (estimated 4000 second execution time).

My question is this: Is there a way to either

a) change the datatype in Virtuoso manually
b) use the Geonames ontology to instruct Virtuoso about the correct types
c) alter my query to more efficiently cast to the correct type

I'm hoping option b is possible, as this seems the most effective solution, because the Geonames ontology correctly specifies the types to all of the resulting predicate's objects.

You can find the Geoname ontology here.
You can test the queries above and your own against our endpoint here: http://18.170.45.162:8890/sparql

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