Translation:The boy, who is running over there, loves the girl whom you admire.
It should accept all of "... the girl [who/whom/that/-] you admire" just to make sure all possibilities are covered.
"Whom" should remain in the suggested translation, not to appease the prescriptivists, but because it helps highlight the distinction between joka and jota.
By the way, even without the relative pronoun present, it's still a separate clause.
It can indeed: https://www.merriam-webster.com/dictionary/that#usage-4. And it is.
It's utterly confusing for non-native English speakers when false grammar is used in a language course. Spoken language is different but it's better to be as precise as possible in the written form. Whom is correct and we should therefore use it. Consistently, if possible.
For everyone complaining about who/whom and prescriptivism: enforcing descriptivism is no better than enforcing prescriptivism. We're all united here with one cause: learning Finnish. For a lot of us, English isn't even the native language, and so, we are struggling with two foreign languages at once. Arguing about the one, that is less important in this course, is pointless. Don't waste your time, when you can learn more Finnish! :)