If they see oil all over the engine bay near the line you left disconnected they will know that caused the issue so make sure you clean it very well. I agree with
@The_Ryan, F em. Dealerships screw people over every day.
The downpipe is a relatively difficult job on this car. If you're new to this hobby you should start on smaller jobs like oil changes, spark plugs, basic modifications, etc. Research what's required first, push your skill level a bit but also don't get in over your head or stuff like this will inevitably happen. Youtube is a great resource, specifically Auto Repair Guys. Good luck!