Learn Arabic (Modern Standard): راجع الملخص ولو مرة واحدة.

Literally, ولو does mean 'even if'. In this sentence, there is an implied verb: 'Review the summary, even if [you review it] one time'. In English, we naturally express this conditional idea of a minimum effort as 'at least'.

If I were to read this out loud with case endings, what case would مرة واحدة take?

It takes the accusative case (Mansub), so it would be pronounced marratan wāḥidatan. Noun phrases that show how many times an action is done act as adverbs, which are always in the accusative case in Arabic.

I recognize the root for 'to return' (ر-ج-ع) here. Why is the imperative راجع (rāji') instead of ارجع (irji')?

The verb here is the Form III imperative راجع, which means 'to review' or 'to consult'. The Form I imperative ارجع means 'to return' (like going back to a place). Form III verbs often shift a basic action to involve interacting with an object, so 'returning' to a text becomes 'reviewing' it.

What structural clues in the word الملخص tell us it means a 'summary'?

الملخص (al-mulakhkhaṣ) is a passive participle from the Form II verb لخّص (lakhkhaṣa - to summarize). You can spot this by the مُـ (mu-) prefix and the 'a' vowel (fatha) on the middle root letter (خ). This structure literally means 'that which has been summarized'.