1. Is original transcript different from original marksheet? - Transcript is nothing but a copy of ur markscars and degress, signed and sealed by univeristy registrar verifying that they are genuine
2. Are we required to send only the highest level education transcripts or does it also include high-school / senior secondary certificates and mark sheets etc? - degree/post degree only no highschool/senior secondary
3. Is this request for transcript only marksheet or Certificate as well? - actually its only for markscard but getter to get the degree attested as well.
4. Why is sending originals risky? Is it that immigration office is required to keep the transcripts and you will never get it back? - I am not saying that its risky or not but first confirm if it will work
Again i suggest you to write an email to detroit visa office immediately asking if original markscard and degrees will be acceptable, i doubt if they will work as notarized copied would have worked for you in that case
I would perfectly understand their need for transcripts as in many countries you can get a fake degree and they wont be able to distinguish , by requesting transcripts they are avoiding additional headache of verifying this with university themselves.
for some poeple notarized marksheets have worked @ buffalo and other visa offices, but since in your case they didnt accept it its better to seek visa office opinion before you decide your next step.
No harm is sending orginals if they will say that it will work and they will retun them to you

Thanks