1. Yes, you can use a HDMI to DVI adapter.
2. I would say you can notice some difference between a TN and IPS panel. And not just regarding colors. At least I can tell the difference. The U2311h uses a 6-bit e-IPS panel so it can show some color banding issues and image artifacts from time to time, but not as much or as bad as TN panels.
3. The Dell U2410 uses an H-IPS panel which is a true 8-bit panel and is also costly to manufacture. That's why the Dell is expensive. Is it worth the extra 200 euros? That's for you to decide. I would simply get the Dell U2410.
4. Not familiar with the Samsung PX2370. The technology aimed at reducing eyestrain is simply an auto brightness sensor. If you use the monitor in various lighting conditions, then I suppose it'll help. But if the light conditions in the room is always the same, then there's not much benefit. The question is how sensitive is it and will the change be noticeably different? This monitor also uses a TN panel.