Restore primary sed+ modified test

[scuba-tech]

TODO:

1. Restore primary sed+ modified test, and during this test have the host machine re-sort (-k 2) the .old checksum to compensate for differences in sorting behavior between individual locations and macOS-vs-Linux

2. Keep the second awk mod check (as a sanity check for users), clean up/refactor slightly

3. Include a note explaining that both diff methods should agree, and if not, they act as failsafes and to investigate any discrepancies as potentially modified (and to send me a screenshot for debugging!)