This was a pretty good plan though. The most interesting part is what exactly the military is cooking with stuff like this. They went out of their way to have a spy serve to death, but still try a super risky corpse retrieval that could reveal the whole plot?
Security of what? It’s not like anyone would steal some ashes from there. And Nemu also got complete access to all territories as a sanctioned corpse bearer, so the guards would let her pass. Also, why would they clean up some burned wood in the middle of the a dangerous forest? Even if they had a protocol of cleaning up the event, it wouldn’t make sense to clean the ashes of the deceased in the very same day.
She said it herself, it's not very well known and the ones who own that title are few and far in between. Besides, it's not that they look down on HER, they despise the function itself - you can compare it to jobs in our world like being a pathologist or better yet a fisherman - no matter how great of a reputation and trust, you still smell fishy.