You can set the origin point in the animation editor. On the left under the toolbar is a button with the target on it. It lets you move the origin point to the middle of your graphic, and can move frame to frame if you animate. (remember, if it is animated, you are only editing one frame, you can right click on it and have it set all the frames to match.)
The origin is the coordinate location for the sprite as well, so you may have to adjust your calculations on spawning and such.
On respawning, you have lots of choices. You can count picked sprites of that type, without actually filtering any to see how many exist, and if it's too few, spawn one.
Or "On sprite destroyed" you could "wait" a few seconds, then spawn a replacement.
Watch the player's score and have them spawn on key values.
There are lots of ways to make them attracted to the player as well, but you almost need to be more specific, as that is a lot to cover.
It could be an invisible sprite surrounding the player, and when the asteroid touching it, it turns slightly toward the player. (the longer it touches the further it turns.)
Or it could always turn just slightly toward the player. (the bullet behavior speed would keep the angle changing, so it would appear to work like gravity.)
There are also plugins for actual gravity points.