This thread has wandered off track a fraction , so I'll throw my uneducated opinion in to straighten it back up
I applaud you for recognizing and admitting the back side of your sheath isn't up to your expectations and your want to improve .
I haven't been doing this long , but like you quickly realized the back of my sheaths were really detracting from an otherwise good thing .
I did two things .
I practiced on scraps and quite honestly you cant beat time and experience .
Secondly I realized that if I'm stitching through 4 or more layers of leather I'm not good enough to use an awl and still achieve really straight lines .
I now drill all my holes and cant see myself changing .
I you drill you must set yourself up perfectly so that the sheath is absolutely horizontal and not tilted . Make yourself a jig out of wood or leather that the sheath sits on in the correct position ( it takes a little time to get this right , and every sheath is different , but 20 minutes spent sorting this out is the difference between success and failure ).
While drilling I don't turn the sheath around and change its position on the drill press , I start at the furthermost hole and work my way towards myself trying to move the sheath as little as possible .
I personally use a stitch groover on both the front and back . The back groove is done after the holes are drilled .
Also something else I've really been trying to get right is starting and finishing the stitching in the right place . I spend time lightly marking out my stitching to make sure it will finish on a full evenly spaced stitch . Sometimes you see where people have run out of room and either have to stop early or finish on a shorter stitch . It looks horrible to my eye and early on I made that mistake a few times . Again spend an extra 10 minutes marking it out before committing yourself . Its small things like that , when combined add up to a huge quality improvement to the overall sheath .
The sheath you made looks really good , if each one you make is fractionally better then soon you will be a master
Ken