Onionskin is working as intended now. It draws what you've drawn above and not below.
Alpha in memory
Now when you go to eraser mode and back to draw mode the alpha is stored in memory. If you drew with 10% alpha and then go to eraser mode (which must be at 0% alpha), then back again to draw mode your alpha will still be like it was before (in this case 10% alpha).
Eraser bug fixed
The eraser decided to be at 0.05 alpha, so I had to make the math 100% accurate. Probably something that was changed with GML. Well, now when you press CTRL + D it goes to 100% opacity or 0% opacity with correct math.