The plushy is adorable, and your wig looks great.
Based on this photo
, her crown has 5 pieces meeting at the cross in the center, not 4. Whether or not you should fix this is a personal judgement call. I don't think most people would ever notice, and among the few who do, I don't think many would really care.
Depending on what you made it from, you can potentially smooth and perfect the white edges and the cross by sanding them with some medium-coarse sandpaper. Crayola Model Magic doesn't sand well, unfortunately. Most other clays sand with no major issues.
Small divot type flaws can be fixed using a bit of jointing compound from the hardware store. Wood putty is another alternative. Regardless, you apply it in excess (leaving a little hill), let it dry completely, and then carefully sand it flat and smooth. Once everything is smooth, you can then sand the entire surface with finer and finer grits until you reach 220 grit. Then after carefully blowing/wiping off all the sanding dust, you can repaint. Your paint job looks lovely.
On some images I'm seeing it appears the crown has a black pillow tucked inside, on other images you can see right through the thing. So I guess that means it's whatever you prefer. If you do decide to make a pillow, I'd recommend using velvet/velour, since real British crowns used velvet.
You're doing good stuff! Keep it up!