Hello Martin,

I am sure people have attempted each of these 4 points but perhaps not in this succession. Here are some programs to try for each stage:
1- Draw Dataset Plugin (Remember to use a different integer value for each ROI, do not use 0 , do not collect $50)
2- Try the program 3dROIstats, that'll give you the number of voxels in each ROI. You can then multiply that number by the voxel size for a volume estimate.
You might also find the programs 3dmaskave and 3dmaskdump useful.
3- If you have the anatomical parent in ac-pc you can use the program adwarp to change your ROI to ac-pc orientation. You can also use AFNI’s Define Datamode --> Write Func button to write out the mask file in ac-pc orientation.
4- I am not sure what you mean by "fractional volume from anterior to posterior" but 3dFractionize might be what you are looking for.

If all these programs are new to you, I suggest you proceed with caution and make sure you understand the output at each stage.

cheers,
ziad