ok mine is sorta done and i cannot stop eating it!! i didn't actually get to the egg part, but i imagine it just brings it to the next level. however, i would say that if you're hesitant about the egg, i would sooner exclude it altogether than to just use egg white. i added potato to give it more of a main course heft. i think the croutons would be fine provided you make them yourself, as suggested by the recipe. that way you control what's going on. i think a crouton would be nice, actually. also, i'm not really a recipe follower so i just added olive oil as i saw fit in order to sautee the huge quantity of leeks. i imagine i probably used at least twice as much (enough to coat the bottom of my soup pot and then to remoisten things when it got a bit dry). but again, soups are not an exact science so i'm sure you can tweak liberally. ie: i decided i wanted more soup so i just added more water and simmered longer for the flavor to really get in there.
as for the seasonings. i used two ENORMOUS leeks. leeks, if you're not familiar, bear some similarities to onions. i also doubled up on the garlic because i love garlic and was using more water. the rest was just seasoned with kosher salt and a bit of pepper. it is absolutely delicious. you don't really need seasonings because simmering the leeks and garlic in the water makes something like a vegetable stock. salt to taste. also the grated parm on top adds a really nice touch and also adds a bit of extra complex saltiness.