Hi Tam,
1.Yes, you can use the past tense after the verb "Imagine".
2. A comma is not required after paper.
3. MCs' should be correct.
4. All MCs'
5. "to support" or "supporting"...the -ing form doesn't allow the "to". In this case "to support"
6. the Olympic gameS ARE held
7. I don't understand what you mean with that question, what are you trying to say?