"She does not eat enough."
Translation:Sie isst nicht genug.
Nicht comes before adjectives and adverbs.
Couldn't "enough" mean adjective "sufficient", adverb "sufficiently", or noun "a sufficient amount" depending on context?
Yes, although the last would be considered a noun phrase I believe.
So how do we know which rules to use?
I don't know of any specific rules in this case, unless you're referring to the use of "nicht" over "kein". This link might be able to sum it up better than I can here. If I were to oversimplify, "kein" works like the English words "no" and "none" while " nicht" works like the English word "not".
So in this example, you wouldn't say "none sufficient" or "no sufficient", but "not sufficient" instead and therefore would use "nicht". Same goes for the other two examples you provided.
You can't use ‘machen’ as an auxiliary verb in German. ‘Sie macht nicht genug Essen’ would only make sense if ‘Essen’ were a noun (capital E) meaning ‘food’: ‘she does not make enough food’.
Negation in German is achieved by simply adding a negative particle (/pronoun/adjective/adverb), no auxiliary is required: ‘I don't see’ → ‘ich sehe nicht’. * ‘Ich mache nicht sehen’ is grammatically incorrect.
In other words, there is no way to translate this sentence correctly using ‘machen’.
The adverb. But I think negating the verb in this sentence would be weird/uncommon (in German). Even in English, negating the verb is more likely to be interpreted as a negation of the whole anyway (i.e. ‘eating enough is something she isn't doing’ rather than ‘she has food in sufficient amount and she is not eating it’).