Translation:I know a great lawyer that can help you.
I agree with you about the "who" part, although there are many English speakers WHO get that wrong. The program definitely should not be marking it wrong though.
As for "ótimo," I suppose it could be translated either way (and if you hover over it above, it gives you both). But there is another word for "good" ("bom") and another word for "excellent" (excelente), so "ótimo" just falls nicely in between there. It also translates well to "great" as an exclamation upon hearing good news, as people who would say "Isso é ótimo!" would say "That's great!" in English more often than "That's excellent!" Perhaps it's overthinking it a bit, but I would just have to say that "ótimo" is the go-to word for anything better than just "bom" that doesn't need that emphatic "excelente" (after the noun).
English slangy-ness kind of makes these words equals...