Just adding to this thread, after 6 months of trying, I got the 3rd key, related items, and the GG within 12 hrs. Here's what worked for me.
Used a premium shield medal and only at grave robber at LP at 836:
250-15 sec jobs-no key
8-1 hrs jobs-no key
25-15 sec jobs FOUND KEY
1-15 sec job firefighting-found slip of paper
8-15 sec job charlatan-found map
2-15 sec job found treasure chest
I feel that I got extremely lucky so it is possible to get a grave robber using 15 sec jobs.