No need to register the copyright; you already own it and have since the day you put pen to paper (or fingers to keys). Registering it doesn't give you any added protection. Save yourself the time and effort. Besides, words don't become valuable until they are published. I have yet to know of a single case where an unpublished manuscript was stolen and published by someone else. Plagiarism is 99.999999999% of the time from work already published. It also looks amateurish and prima-donnaish to have a copyright when you go to submit. Read several of the threads here on AW on the issue. People well more knowlegable than I have already weighed in when others have asked.
A word about crits. Nobody is going to tell you it's crap, so long as you have the basic rules of writing down (which, based on your posts, you do). However, some people are sure to tell you it doesn't work for them, for reasons X Y and Z. You might not agree with them, and that doesn't mean they are calling it crap. Where I have gotten the most frustrated with SYW is when I think there is a magical path by which everyone who crits is going to be blown away. There is also a tendency of critters (I've fallen in too when critting) to overanalyze, especially with very short pieces. I sometimes feel I have to give constructive input on a 500 word slice when the same 500 words in a larger piece would work just fine and I wouldn't think twice. All that to say: in the end, YOU control the story, and not one word will change unless and until you decide it does. Consider the merits of all input, but reserve the right to acknowledge and move on if the advice doesn't work.
