Some thoughts from a long-time vegan (warning for a long post)
Re stepping on snails and the like: The Vegan Society talks about not using or exploiting animals "as far as is possible and practicable". The "practicable" is really important for those living a vegan lifestyle. You can't reasonably avoid killing insects when walking, rodents are likely killed when the grains we eat are harvested, etc. There are trace animal products in many items that don't have vegan alternatives (bike tyres, for instance). Many people have jobs that may require them to use animal products in a small way (wearing a uniform with leather or wool, for instance). Some vegans, including myself, take medicine necessary for quality of life. In my case, the ingredients are vegan but it was surely animal tested. Now I could survive without it but before I started on it, I was left with a gynecological condition which sent me to the hospital on more than one occasion. I was anemic and required blood transfusions. In all of these situations, a vegan must decide what is "practicable" for themselves.
On the idea of every vegan having chosen to eat something non-vegan at some point...after a sort of breaking in period, I've not generally found this to be true. There are times when vegans buy something that's not vegan accidentally and may choose to eat it instead of it being wasted (foe instance, they switched the chocolate sauce at my daughter's favorite restaurant to one with dairy which we discovered after it was already at our table.) Each person needs to decide that for themselves but some vegans would choose to eat it anyway since throwing it away wouldn't save any animal products from being used. As far as vegans choosing to just sit down and have a roast beef sandwich, that's not so common.
On veganism being the most ethical diet, I'd say that you can't measure that because there are junk-food vegans and raw vegans and vegans who don't use palm oil and all sorts of other varieties. In the same way there are larger variations in omnivorous diets
Regarding using a vegan label. I do think it's important not to say that you are vegan and then (intentionally) consume animal products, especially in a food service situation. It's not about purity or anything but about not muddling things for those who choose not to eat animal products. That is, if you eat a cheese sandwich, people might think that all "vegans" do.
All of that said, I would encourage people to do what's right for them and not to worry about it being all or nothing. We have some friends who are interested in veganism but feel daunted by the idea. They are making small changes for now: non-dairy milk in their coffee, eating veg curry instead of chicken, etc. If you feel like you could be vegan except for milk in your tea, then do that. If the thought of not having turkey at Christmas is too much, then have a "mostly vegan" diet and eat the turkey. Not to rip off Tesco, but every little helps :)