Is there a psychopomp for robots?
Nope, there isn't. 
A question about the Guides whom Annie was showing talking to in Chapter Nine... Two of them are easily recognizable as Hermes and the Grim Reaper. The other three are an ancient Egyptian figure, a two-faced figure with a Hindu look, and a snake-like being. Are they also "real" mythological figures (in the sense of being ones from books on mythology, rather than inventions of the webcomic)?
The ones people think are the Grim Reaper and Hermes are actually Ankou and Mercury (Hermes wasn't a psychopomp). The Egyptian is Neith, the Hindu is Agni and the horned guy is Veles. They are all actual myths. 
Wasn't Mercury basically the Roman version of Hermes anyways?
They shared many similar characteristics, yeah, but they arent the same. 
I thought that Hermes was a psychopomp; he was portrayed taking the souls of the dead down to Hades. (He does it with the souls of Penelope's suitors, just slain by Odysseus, at the beginning of Book XXIV of the Odyssey.)
Looks like you are right about that. In that case Hermese and Mercury are interchangeable on that page since I don't label them specifically. 
Is the fact that you go with one or other Guide dictates your afterlife? I mean, do all people go to the same place eventually or they get separated?
Everyone ends up in the same place. How you get there may vary. 
What are the factors that determine which psychopomp's jurisdiction a soul belongs to? The ones that don't remain to be seen, that is.
Mainly location, or origin of their family. 
My guess is that psychopomps can make themselves visible to anyone they want to, though they usually don't.
That's right. 
Under whose jurisdiction do you fall? I mean, which psychopomp would theorectically come for you when you die?
Not sure, really. I guess some Catholic one but it'd be a coin toss if they were English or Spanish. 
Does Annie know which of the Guides will come for her when she dies?
It's not been said. 
Are the psychopomps themselves immortal? Are they vulnerable to anything?
They are immortal, but could probably killed through etheric means. 
Why can't psychopomps interfere with the world of the living? What's to stop them?
It's not really their place to mess things around. 
Is Ketrak the only psychopomp for insects? How could he possibly manage all by himself?
He can be in many places at once. 
On , why does Annie believe that there's some correlation between Muut and the Shadowmen? Do they need guides like humans do when they die?
Coyote is Native American, as is Muut. The shadow men were created by Coyote. 
He's an important guy, so it wasn't so hard. 
Is there more than one psychopomp for each culture, or is there just one? For example, is Muut the only one that picks up Native American spirits?
Nah there's loads 
Can a psychopomp die? How about a god (like Coyote)?