Here's a way without indirection:

Class MyPackage.MyClass
{

ClassMethod MyMethod(p1 = 1, p2 = 2, p3 = 3)
{
    Write p1,"-",p2,"-",p3,!
}

/// do ##class(MyPackage.MyClass).Test()
ClassMethod Test()
{
    Do ..FromJson({})
    Do ..FromJson({"p1":"first value", "p2":"second value"})
    Do ..FromJson({"p1":"first value", "p3":"third value"})
    Do ..FromJson({"p2":"second value"})
}

/// do ##class(MyPackage.MyClass).ArgPosMapping("MyPackage.MyClass", "MyMethod")
ClassMethod ArgPosMapping(class, method, Output map)
{
    kill map
    set formalspec = $$$comMemberKeyGet(class,$$$cCLASSmethod,method,$$$cMETHformalspecparsed)
    for i=1:1:$ll(formalspec) {
        set arg = $lg(formalspec, i)
        set map($lg(arg, 1)) =  i
    }
    
    set map = $ll(formalspec)
}

ClassMethod FromJson(json)
{
    Do ..ArgPosMapping($classname(), "MyMethod", .map)
    Set iterator = json.%GetIterator()
    Set position = 0
    While iterator.%GetNext(.key, .value) {
        Set position = position + 1
        Set arguments(map(key)) = value
    }
    
    Set arguments = map

    Do ..MyMethod(arguments...)
}

}

Is it service or operation? Please elaborate on your configuration.

Because you have XML error, I think there are some additional transformation(s) taking place.

Have you tried checking Stream at line 3 offset 120?

Documentation.

Check %SYSTEM.Mirror class, it looks like what you need.

Write class method - sqlprocedure to expose the data you need via xDBC.

Doesn't CORS exist to prevent exactly that?

Check that origin/referer headers in your OnHandleCorsRequest are what you expect them to be and all would be good.

E=D should work.

You don't need to throw an exception, just return error %Status from handler method.

I use Pycharm  for complicated projects and debugging and bash for quick scripting.

Also check PythonGateway - it comes with integrated Python interpreter right in your IRIS terminal.

Tip.

Metadata does not change from one row to the other and accessing it takes time, so removing it from the row loop would improve the execution speed:

Class User.Test
{

Query TestQuery() As %SQLQuery
{
SELECT * FROM Sample.Person
}

/// do ##class(User.Test).Test()
ClassMethod Test()
{
    for method = "EveryRow", "Once" {
        set rSet = ..TestQueryFunc()
        
        set start = $zh
        do $classmethod(, method, rSet)
        set end = $zh
        
        write $$$FormatText("Method %1 took %2 sec.", method, end - start), !
    }
}

ClassMethod EveryRow(rSet As %SQL.ISelectResult)
{
    
    set tResults = []
    while rSet.%Next() {
        set tRow = {}
        set tMetadata = rSet.%GetMetadata()
        set tColumnCount = tMetadata.columns.Count()
        for x=1:1:tColumnCount {
            set tColumn = tMetadata.columns.GetAt(x)
            set tColumnName = tColumn.colName
            //do tRow.%Set(tColumnName, rSet.%GetData(x) )
            set $PROPERTY(tRow,tColumnName) = $PROPERTY(rSet,tColumnName)
        }
        do tResults.%Push(tRow)
    }
}

ClassMethod Once(rSet As %SQL.ISelectResult)
{
    set tResults = []
    set tColumns = ""
    
    set tMetadata = rSet.%GetMetadata()
    set tColumnCount = tMetadata.columns.Count()
    for x=1:1:tColumnCount {
        set tColumn = tMetadata.columns.GetAt(x)
        set tColumnName = tColumn.colName
        set tColumns = tColumns _ $lb(tColumnName)
    }

    while rSet.%Next() {
        set tRow = {}
        for x=1:1:tColumnCount {
            do tRow.%Set($lg(tColumns, x), rSet.%GetData(x) )
        }
        do tResults.%Push(tRow)
    }
}

}

Results for me:

>do ##class(User.Test).Test()
Method EveryRow took .017803 sec.
Method Once took .01076 sec.

Make database RO?

Do ##class(Ens.Util.LookupTable).%ClearTable(tableName)

By default Ensemble service is run under System account, which does not have user environment variables.

You can either specify system variables or run Ensemble service from a user account.