Thank you both for the replies and explanations, it is all very much appreciated.
Here is the rough thought process I tried before asking for help:
Summary
If I considered な coming from ‘na-adj’ (or na-noun), then what does の mean?
I tried to brute-force the uses of の that I’ve encountered:
- N1のN2 - can’t be it, as we have attached な to N1 already and a Noun takes one or the other.
- の normaliser - but why would we normalise a N1な? what would that even mean? nope.
- explanatory の - I’ve only seen as a sentence ending particle, and here we are followed by ダメ.
At this point I threw in the towel.
I don’t think I had seen this before so initially I wasn’t sure how to make sense of your reply sorry, but now combining it with @ChristopherFritz it makes sense 
Is this の the same as the explanatory の? Or are they different?
I ask mostly because up until now I had thought explanatory の had to be a sentence trailing particle, so I’m wondering if that was wrong, or if this is a new/separate particle that just happens to be spelt the same.
I think it is making sense to me.
This usage of の is extremely interesting!
but going to try and catch up as best as I can! I’m not sure if I can explain this very well, but I seem to be able to understand the main points of a sentence usually, like the noun and the verb, but often struggle with the grammar (I do know this is a weak point of mine). Am I best trying to look up each grammar point and try and learn or just try to get through this and continue through Genki and other resources? Though honestly, in the Manga I do struggle knowing how to separate words and how to google what grammar point I’m looking for.

