Interesting you should use Evening Grosbeaks in your example. Since they are a declining species, it is an important bird to report on eBird. While we all should strive for accuracy, being off by a bird or two (or many more if estimating a huge flock, say of Red-winged Blackbirds) is probably not all that significant. I think most of us can estimate a flock of 25 birds as compared to one of 50 or 100.
As for your second example of the Enosburgh Town Forest, you can report a "Travelling Count" that will include all the birds you saw on your walk. The count will include the distance you walked. Since this is something you do frequently (and probably for about the same distance), once you have that location plotted in your eBird account, it will be a simple matter to enter the data. If you would like help with the instructions on how to do that, email me separately, and I'll see if I can help.
I think you questions are good ones and similar to reasons others have not eBirded.