Since you're manually getting data to add to the Store, you should setMode(ComboBox.LOCAL) in your ComboBoxConfig.
In your callback, try adding data to the Store as follows :
String[][] results = ...
for (int i = 0; i < results.length; i++) {
String[] rowData = data[i];
Record record = recordDef.createRecord(rowData);
companyStore.add(record);
}
companyStore.commitChanges();
I'll look into why your code that uses MemoryProxy doesn't appear to be loading the Store.
Sanjiv