To be honest I would expect dates on a Dutch site to be shown in dutch format, just like I expect dates on American sites to be formatted in the US way.
- Date display. I prefer to show it in the browser preferred format, but in the end what you say is also OK. It's a very minor issue.
- Date input. I would never force a format to the user, it there is any chance people from different locales can access the web. The correct way, IMO, is "input in the preferred browser format, send data as ISO". Any other combination would cause errors, e.g. "input in the preferred browser format, send data in that format" and you'll get wrong dates in the backend, or need to do a lot of gymnastics to avoid them. "input in server locale, send data in any format", and you'll get wrong dates from people used to any format that is not your server locale that happens to validate in multiple locales. "input in ISO, data send as ISO", you'll get users upset because "YYYY-MM-DD is stupid, use MY locale instead".
I'm in agreement with showing/inputting dates in the users' regional setting.