All great points. I've learned from experience to pay attention to course markings/signs and not those 'leading you'.
I was running a half 10 yrs ago and running beside a Colombian lady who didn't speak English. A sign showed the HM going straight, while the shorter distance turned. The Colombian lady turned, while I kept going straight. Couldn't believe it! You get in some of these races with international runners and they may not be able to read the signs.
You don't even need to know Mercedes ahead of time cause it's so well marked/signage.