Also, "Robot Egg Man" sounds much less threatening than "Robot Eggman". Especially because it could be read as a man who makes robotic eggs. Which could be pretty cool, depending on what they did, but global domination seems unlikely.
The US/UK guides are nothing more than fan-fiction, as they were written by people that were not involved with the games. "Stone Sonic" is a good example of this.
I think the Japanese calling it the Death Egg is really about as bad--more likely they were not provided a name for it from Sega and somehow didn't make the Star Wars connection. Isn't the Death Egg boss of S&K also called Robo Eggman? What do the guides besides Sonic Jam's call it?
"Giant Eggman Robo" according to SMC. Sonic Jam calls it Eggman Robo and occasionally gets the "giant" (kyoudai) in there too, so it's hard to tell if it's part of the name or just an adjective. Not especially conclusive, and not as interesting as arguing over the proper names for Mecha Sonics, since the Robo Eggmans are not really "characters." But it feels to me like the same sort of "series" of bosses sharing a similar name, and Death Egg doesn't really fit, especially since it's also definitly the name of something else.
Just for the record, I just got an actual Japanese Sonic 2 (in startlingly good condition) and discovered for the first time that the plesiosaur bot I had been calling "Saura" is actually "Zaura". Apparently the ゛above the サ was obscured in the Sonic Jam files I'd been using up until just now. I feel so ashamed.
I think there might be some thread out there that changed its topic right in the first post.
Is there an actual difference between "s" and "z" in japanese?