Arlington: I've discovered that I'm a better soldier than commander, as I miss writing regularly and have come to loathe spending most of my time overseeing projects. How can I present myself for a position below my qualifications without looking like a slacker? Or as someone unable to handle heavy work?
This is common. Prospective employers, however, are not likely to worry too much about you being a slacker -- they may, instead, worry that you will try to be the boss. Regardless, don't worry about it too much. You explained your motivations to me succinctly here; you can do the same in your cover letters and interviews.
Washington: I have been out of school for two years and currently work at a small nonprofit. I am fairly satisfied with my job. However, there is no room for advancement as there are only three other people working with me. I have been here for a little over a year. I am in an administrative position, but this isn't where I want to be forever. With little experience in anything other than administrative work, should I begin to look for something else?
First, redefine your concept of "advancement." It doesn't necessarily mean an official change in job title, especially this early in your career. Now that you've settled into your basic job duties, start asking your supervisor for more responsibility. In an office as small as yours, it seems unlikely you'll be turned down. If you perform well, eventually you will earn a full-fledged promotion at your current job, or you'll be able to translate your new, if unofficial, skills into something other than a lateral move.