Looks like a logic error in the script:

    // delete monitored item, but temporarily break it's monitoredItemId
    items[0].MonitoredItemId += 0x1234;
    DeleteMonitoredItemsHelper.Execute( { SubscriptionId: MonitorBasicSubscription, ItemsToDelete: items[0], OperationResults: new ExpectedAndAcceptedResults( StatusCode.BadMonitoredItemIdInvalid ) } );
    items[0].MonitoredItemId -= 0x1234;
    DeleteMonitoredItemsHelper.Execute( { ItemsToDelete: items, SubscriptionId: MonitorBasicSubscription } );

both DeleteMI call fail like this:
DeleteMonitoredItems().Response.Results[0] = Good (0x00000000), but BadMonitoredItemIdInvalid (0x80420000);
DeleteMonitoredItems: Results[4660] is not good: BadMonitoredItemIdInvalid (0x80420000)

note: 0x1234 = 4660, thus the first call seems to delete a different item,
and then the second call Fails as well, because the first call did
delete the MI#4660