Could one argue that parental discipline constitutes mental/emotional abuse in certain cases? At what point does punishment (ignoring physical punishment for this question) become abuse?

That parental discipline may constitute a form of abuse depends entirely on what you accept under the label of "discipline". Consider for example a family in which following some religious practices - like preying before supper, or not eating certain kinds of food - is considered as part of a discipline that children are obliged to follow, and a 10 years old child (that is, someone who is cognitively able to take at least partly autonomous decisions on her moral preferences, even is she still doesn't have reached the institutionally established "age of reason", usually 16, 18 or 21 according to the countries) who refuses to comply. In this case, I would consider a sanction of her behaviour as a form of abuse. Punishing her for not complying to a rule she doesn't want to endorse because she finds it incompatible with her ideas and moral feelings is a form of abuse. Abusing children means prescribing them a system of rules of disciplines without taking their stance and thinking about what is...