Coherence relates to how understandable and intelligible a text is to the reader. For example, the use of paragraphing, headings and topic sentences helps achieve coherence by organising the text into logical sections and clearly indicating what each section will cover.
Cohesion relates to the fluency of the text and how the text 'hangs together'. For example, anaphoric referencing allow a sentence to refer back to a subject through pronouns that avoid needless repetition hence improving the fluency of the text and making it more cohesive.
Repetition can however be used to achieve cohesion. Repetition is an example of lexical patterning, which relates the sentence back to the same key idea/lexeme.
As for inference, most questions in VCAA exams will ask how the text relies on reader inference to achieve coherence. In plain English, this means "What does the reader need to know to understand what the text is saying?"
For example, in the line: "Our lawnmowers are the Mercedes in the business." relies on the reader knowing not only what a Mercedes is, but also that the car has a reputation of high quality, thus making the inference that the lawnmowers are also of high quality. If a reader does not know what a Mercedes is, then the text becomes incoherent as the intended message is unclear.