So this was finally resolved yesterday evening by me security admin. The firewall needed to have a rule set up with a ficitious address to redirect the traffic to the correct port for the SQL server/cluster. It makes me think that the dboptions array is either redundant as it's directing traffic at a random port, or the syntax is wrong in my config.php; or like Ken said, perhaps there's an incompatible character. The front end seemed like it wasn't 'paying attention' to the settings in the config file where database server ports were concerned and just doing it's own thing.
I guess the lesson here is that if you're using a corporate network, like the one in my local authority where the web server and db server are separate, you will need to get creative with firewall rules. If anyone has any suggestions for how to configure the config file differently, then I'll happily post a redacted version of the code up here for your input.