Meaning 3 for Biblical Hebrew נָא is not due to a misunderstanding of the Hebrew on the part of the targumists. It is rather a discourse marker as is "now" in English and other languages. Compare, for example, the substantial semantic and stress-position differences between "come nów!" and "cóme now".
It is quite possible that in some cases JlAtg מִכְעַן means "therefore" rather then "from now on".