I would suggest the use of a fork instead of a spoon in mixing and loading the substrate.
It leaves the mix light and airy, checks the moisture content and prevents clumps, and forces
you to be more careful in loading the jars so you don't get as much contam of the inside rim.
I would also suggest using a paper towel over your thumb in cleaning the inside edge of the jar.
Just an extra step of caution I know but oh well

I haven't read your writeup in detail but in skimming it looks good.
Sometimes a different point of view will help the understanding and improve odds of success.
Thanks for helping