You can shroud and get the quest at a lower level, but unshrouding and going to the buried earth *graves* in the lower-level zones will not work. Apparently it does a check for the toon's current level when you click the shovel. (I did not try staying shrouded as that defeats the idea.)
The mobs that spawn in Field of Scale are all undead and /con as "weak" (they quad but miss often and hit like other weak mobs in the zone). Only the "a tattooed zombie" will bring the tattooed skin, so many people just leave the mobs up (rude). The piles will repop even with a previous spawn still there, and if you have IVU on you can use the shovel without the previous spawns attacking you. (Using the shovel should break invis/ivu but doesn't seem to.)
The lockout on the digging task is 18-hours from when you get the task, although you do not get the actual lockout unless you finish the task. (This is the same mechanic on all the timed tasks and why it appears to vary -- people take different amounts of time to complete tasks.)
You must complete the Digging Their Graves task at least four times to complete the The Hunt for Tattooed Flesh task -- maybe five but I cannot see the final step yet.