The point I am trying to make is if Northern aren't providing any kind of proper response to my issue, then how should I trust that it is being (or going to be) investigated?
Having looked at the FOI email chain, they have responded with what you had requested, namely all emails in the time period specified.
They have chosen not to engage any more with you based on your original direct email exchanges with them, which is mentioned in the FOI response.
You have raised your issue and told them what you perceive the issues to be. I don't believe they owe you anything else and I can also see why they have chosen not to engage further based on your communications with them. A different approach to how it was originally raised, may well have yielded a more positive response, but sadly that horse will have bolted now.