When you float the bucket, you are generally backblading to smooth something or with a steeper angle, to drag something. I grade everything I do by floating it with the loader bucket (b2630) and I can get it spot on. I do have lot of practice however from bigger machines. I would never consider wasting my money on a box blade. Loader is way quicker and much less hassle. I can work circles around a box blade with a loader and you would not know the difference in the finished product. I know I'll get an argument on this, but I'll stand my ground!
Your 3030 is the same as mine. The float works just like any full size machine. I floats whether you are going forward, or reverse. The only time I float when going ahead is when I'm on an asphalt or concrete surface and want to scrape it clean. If you float going ahead on dirt, the bucket will either dig in or ride up - you'll never go level. Much easier to set it, go real slow, then feather the bucket as you go to maintain your grade.
To answer the rest of your questions, your loader is not self leveling, and the loader floats up and down, whichever way it is "pushed".
Summary: Float at about a 45 degree angle or better to drag material in reverse. Decrease to about a 30 degree angle and you will start to smooth/spread in reverse. As the amount of material behind the bucket starts to wain, just keep rolling back the bucket until just about flat and you will just taper it right down and have a pretty smooth spread. May have to repeat a couple times to get it just right. I never float going ahead on dirt to dig.