Raoul 🇳🇱🇬🇧(🏴)
1. It's not that much of extra costs, you do not need to rewrite the website for each country.
2. It is possible, technically Finreach sends the wish to change your bank details to all the companies taking money from your account, so you don't need to send all that letters. They just take your work you would need to do, nothing more (and e.g. N26 has only a license in Germany but still offers the Switching service for Austria thanks to Finreach).
3. As I said: Some of you wouldn't use it, but some people would. A popular example is bunq itself: bunq.me offers Sofort and iDEAL, so if you have a bank account outside of the Netherlands without Sofort, you can't pay someone with bunq.me :) And some shops do really only allow Sofort or a normal transfer (which takes longer).
5. The same size would be the best solution, but if bunq doesn't want to do that it should be based on IP, and mostly (if you are not online via VPN) the IP is telling the truth about your country.