When in edit mode press Right Click > Decimate curve, Then in the bottom left corner click on the foldout (Or press F9) and set the decimation ratio.
This will keep the original shape more precisely than deleting every other vertex.
For part a) You can selected every other vertex by the select checker deselect method. In edit mode select all verticies that you want to "unsubdivide", then through the Select menu, choose Checker Deselect (Note this has . Be cautious of selecting the end points of your curve.
However I am sure there may be better ways?
I had the exact same issue until I realized I shouldn't use the Annotation tool but a real grease pencil layer.
Add > grease pencil > Blank
You then switch from Object Mode to Draw Mode and switch the Stroke placement from Origin to Surface
I found the answer in another post.
Youll find it in Object Mode, Object, Convert To
Im just find the same problem with those tutorials xD
Source:grease pencil strokes to mesh is not available in 2.8 release
Choose the stroke. Go to the object properties(its the cube symbol on top of the modifier properties). Then go to viewport display and check in front. But that would place the stroke in front of everything else. I think that it would work for that what you want though.