Hi Zheljko,
Thanks for taking the time to submit your observations to eBird, so other birders and scientists can benefit from them! Please be aware that eBird recently added a new "Help" tab to its website with a comprehensive list of responses to FAQs. There is also the ability on that tab to post questions for eBird staff or other eBird users to respond to, and there has always been an ability to e-mail eBird staff with questions. In any event, I will try to address some of the issues you raise that have not been answered above.
1. Regarding entering data for distance traveled, that is important but a rough estimate is sufficient when you lack more precise information. The reason it is important is because it gives an indication of the density of bird populations. Seeing 20 Great Tits at one location would be unusual, but if you traveled 8 km, it might not be. You likely have a good idea of whether you traveled 100 m, a half kilometer, or 2 or 3 km, without having to make a precise measurement.
2. You mentioned that on your lists the name White-winged Scoter was used rather than Velvet Scoter. That is a difference between American and UK English terminology. You can change that in your preferences. There is an option for you to specify what language common names should be translated into, and if you want UK English terminology, just select the option for "English (UK)".
3. Regarding reports of birds that are clearly misidentified, these will ordinarily be caught by eBird filters that require an eBird reviewer to confirm unusual sightings before they appear in the database. However, eBird only recently expanded beyond the Western Hemisphere, and the process of fine-tuning filters and finding reviewers for the Eastern Hemisphere is a work in progress. Once the filters are fine-tuned, they should be rerun on the observations in your area and the errant sightings you reference should be eliminated.
Hope this helps,
Jim