Sailing downwind is a lot harder than upwind as a beginner. I think you are confusing some of your terminology. Having a roller furler has little to do with having a spinnaker.
My advice for you would be to avoid anything complicated like wing-on-wing or using a whisker pole at this point in your sailing career.
When sailing downwind:
1) Avoid sailing DEAD downwind for now (wind directly behind you). Try sailing a broad reach instead.
2) Yes, your sails should be fairly far out. Your fear of the boom coming across the cockpit is and should be a real concern. An accidental gybe could injure somebody so make sure your heads are down and as your sailing you need to do you best to simply avoid the accidental gybe to begin with. You might consider rigging up a preventer
3) Sailing downwind is difficult and you need to be careful. It's not dangerous to have the boom out that far in and of itself. Sailboats routinely sail with the boom out perpendicular to the boat, just see #2. The more you sail the better you'll get at avoiding accidental gybes.