First - either method:
- Send iPhone to repair (lasts 9-17 days);
- Exchange the iPhone for a new one, costs 29 euros.
Will not be a repair and return of your original phone. You will receive a refurbished or new phone, it just depends on what they have available at the time.
I doubt the dust is your fault. The screen is a bonded assembly of back light, display and front glass. Are you sure it is dust and not points created by developing separations on the bonded components i.e. small little air bubbles.? In either case you can ask them not to replace it if they find the problem to be your fault. But to make that request you will have to go with option one "return to them and wait for the new on to be shipped to you" - 17 days without a phone is a long time unless you have another phone to use.
The 30 euro method is the most convenient method. They send you a replacement "overnight" that comes is a box that has a prepaid return label. So while this method cost you 30 euros you don't have to pay any additional shipping.
I exchanged mine using the $30 dollar method here in the USA and was very satisfied with the whole process and had a new phone the very next day and never was without a phone.
Sure you run the risk of getting stuck with the 250 euro charge but talk to them and see if you can get them to cut you some slack and allow you to back out if they make a finding that it is your fault. My personal opinion is they won't sting you. As I said earlier the chances of dust getting into an iPhone is pretty **** good but getting between the display elements and the front glass slim to none from anything you did.
Good luck whichever way you decide to go.
I looked up your town, 20,000 residents so I guess it is safe to assume there is no Apple store there. I live in a town of 75,000 in south west Utah and the closest Apple store in in Los Vegas (150 miles). But you might be able to go to your provider and just ask them to give an opinion of what Apple will do.
My best suggestion is to call Apple again and chat with them about giving you a way to back out if they determine the fault is yours. It depends a lot on the person that answers you call but most of them understand how sensitive iPhone owners are to imperfections and most of the underpaid Tech Support folks know how much 250 euros means to the average user. Keep in mind that they are just as interested in keeping you as a happy long term Apple user as you are to remain one. So give them you best "Poor Me" story.
"Annoying" - I can sympathize with you. It would drive me crazy also in fact most iPhone users are extremely sensitive to any cosmetic damage to their prized iPhone.