Without context, the difference between "The person I had seen." and "The person I saw." Is the same as "The pizza I had eaten." and "The pizza I ate." I think both tenses should be appropriate for these translations.
If you think something should be accepted then you should report it via the flag :)
I had to look up these grammatical concepts, but from what i understand the simple past "I saw" refers to a specific time/place/etc such as "the person i saw at the park/last night" where as "person i had seen" is more ambiguous. This is rather confusing to me as well so i may be wrong or not explaining it correctly
"The person whom I saw" should be accepted, reported on Nov. 13. 2017.
Past tense adds ㄴ/은, present tense adds 는 and future tense adds ㄹ/을. So because it's 본 not 보는 we know it's past tense.
When conjugating a verb with a future, present, or past modifier the modifer is ㄹ/을, 는, ㄴ/은 respectively