I generally do a mixture of filler sprites and final art all throughout the development. I find that a lot of my animations are dependent on specific timing, so having a final sprite done to fit in place for testing is a lot easier for me to program around. Generally I just use plain boxes until I need sprite or animation to solve some programming puzzle, then I make that real quick (well, as quick as I can) and just toss it in.
I find that it's a pretty bad workflow, though. Stopping and starting all the time is a pain. I should probably just do stick figures approximating what the final animations will be, but I'm too concerned with how things look.
I'm sure with more practice I won't need to have a sprite in front of me to know how it's going to behave. And I could probably streamline my workflow by making mockups and a bunch of prototype art before I ever start coding.
As for the animator, click on your sprite and navigate to the Animator bar. Your animations are organized by name and angle. When you first place a sprite, that frame becomes Angle 0 under Default. This is "facing right" so you'll probably want to import your right-facing frame when creating the sprite.
To add a frame to the angle, Right-click in the frame window and add one. It will show up as blank, so double-click it to edit that frame.
Once you have more than one frame of animation in your sprite, the movie-strip will show up in the image editor and you can navigate through the frames for the current angle without having to open each frame separately.
Right-click an animation name to add a new angle. For instance, if you wanted to make a left-facing frame, add Angle 180. The new angle will show up, but the frames will be empty. Right-click where the frames should be and add a new frame. Double click the empty frame to open the image editor and import your new frame.
To make a new animation, Right-click the animation name window and select the appropriate item. You can rename an animation by clicking the name to highlight it and then typing a new name into the Properties bar.
To change the looping or speed of your animation, click on an angle and the options will show up in the Properties bar as well. IIRC the numbers that go into the speed are in frames per second.
I don't really know what sub-animations do, I haven't used them at all.