A3 S3 Sportback Roof Rack no Roof Bars?

Have a look at the Thule website. I'm pretty certain they will do some suitable items.

I'm pretty certain that you can, see Audi Accessories. IIRC the roof bars are must if you have panoramic roof. If you dont have the PR then i'm pretty certain they have something you can use.

Yes, sure is possible as I've got them. I use them to carry my bikes and also for roofbox duties when needed. Got all mine from roofbox.co.uk.

Really a 2 man job to fit as really you don't want to have to remove the roofbox first and then the bars. I just unscrew the roofbars and then lift the whole lot from the car. Roofbox have package deals on bars and boxes and also throw in a hoist which means you can store the box hanging from the garage roof so it doesn't take up floor space.