For separating regions see:
That said, this is just a diagonal, so open the image:
see that the diagonal is already drawn in one half:
Drag it into registration:
Ungroup the geometries which need to be worked with:
Draw in the diagonal:
Duplicate it twice and drag the duplicates into registration:
Join Vectors
Yes
Node edit to clean things up:
d
Done
measure the inset needed for the surrounding circle:
select the circle:
duplicate it in place:
resize it by the noted dimension:
Done
Select the geometries which are needed for each half:
copy-paste them and drag them off to one side:
Use Trim Vectors:
to remove one side:
OK
Use Join Vectors to close:
Yes
Repeat for the other side
OK
OK
Join Vectors
Yes
Drag things back into registration, grouping where appropriate:
Delete what is not wanted:
Assign things to appropriate layers and cut as desired.
Attached as a v8 file.
VMM-268_v8.c2d (1.7 MB)