I really liked choice #1 but I don't think its best if you want to use her as a reference. I voted for #5 that way if you were done with your own responsibilities you could then offer to help her and just say I had 3 other projects due Friday and I just got finished with them and would be more than happy to help you. Then either you'll do it or you won't but you'll look good for remembering and offering to help her.
i'm glad it worked out. when i first started to really get into my career, my life was like that - always working, always juggling a hundred things. it wasn't good. i felt like i lost four years of my life!! i would have suggested being honest - you're bogged down, you want to help but here are your limitations and you can contribute under xyz circumstances (such as the assistant starting it and you picking it up later, or if some other things were taken off your plate). i'm always really direct with my boss and honest, and he i believe respects me for it. stick to that - as long as you're direct, professional and have a solution ready they should receive it well.