Sorry, but Kleiner is taller than Magnusson in-game, the reason you might think otherwise is because Kleiner is always hunched over. And my comparison of Magnusson's and Slick's hair line and bushy eyebrows still stands, as does the lack of mustache on Magnusson...
Yes he is, Magnusson's hair line matches the hair line of Slick in the photo from Keliner's lab, they also both have bushy eyebrows. If Valve wanted to make him look like the mustache guy don't you think they would have given Magnusson a mustache? Really, the guy with the mustache looks a lot...
I'm not sure if I would particularly like to go to another city in Episode 3. Although, seeing one in the distance from the helicopter while on the way to the arctic may be interesting, but since we're still pretty close to City 17 I don't think there are any around.
Yep, Magnusson isn't the guy with the mustache, he is Slick on the far right.
I think Mossman was telling the Combine to raid BME when she went into that room. If you follow her into it she is quick to tell you to leave.
Laidlaw said in an email that the Combine have never been to Xen. We don't know how the Combine found earth or how they invaded other than that the Citadels suddenly appeared inside the cities. The rest is only speculation.
But the whole point of Source is that it doesn't have limits and can continually be upgraded. Since Valve is trying to get the Episodes out faster than if they were making a full game they don't have enough time to upgrade the visuals as much as they could.
But it is obvious it's supposed to be Black Mesa. The commentary even says the scene was supposed to evoke the past, what else in the Gman scene could they possibly be referring to in the commentary other than Black Mesa?
It doesn't matter if it's furnished differently. The animations for Gman in that scene are titled "G_blackmesa_blabla" it's obviously supposed to be Black Mesa.