MINERVA - read-only archives

I'd be careful about that, Kenny, because "looks like somewhere on Earth" is such a broad term as to be more or less useless, in my opinion, since the variety of places on Earth is so broad as to cover just about any natural habitat in which humans could survive unaided. Indeed, it's the inverse of an episode of Stargate: SG-1 in which they find themselves on "an ice planet" ... which turns out to be Antarctica. My first impression, once I'd gotten the idea that it wasn't the Combine homeworld again, was "a fourth dimension altogether."

I got from context that it was near Minerva's location. Close the portal, because they're coming for him/her.

Minerva states that there are some sorts of ancient artefacts there, presumably capable of being weaponised. Thus, it is plausible that they've already opened the portal earlier (It'd be a strange coincidence for the very first activation of the portal to occur whilst you were down there). Thus, they probably sent a few dropships there to either do some recon, or to transport some of said artefacts back to Earth.

Also, I feel this may have been mentioned a bit in the text before the credits.
"We are alone in this battle, so should we dare to take up their
ancient, abandoned weapons and become like gods ourselves?"

I'm not saying that 'ourselves' refers to the combine, although it could. More likely, it just a nondescript group, be it humans, Xenians, some other race, or just all sentient life in general.

Someplace Else took place on Zen.

Not a time portal. Most likely a world that is hard to transit to (distant from most?), hence the reason for the combine to make a portal on earth instead of on their home world. Probably a civilization that has long been dead. Possibly references to and/or artifacts from the civilization/planet on other planets/worlds. Maybe Minerva has not been there before but knows of it.

Certain: We will go there.

It does indeed look as if they're comparing the two worlds. The real question is what they're looking for. A specific location, or a place meeting certain criteria? What I find odd is that, while the end of the portal we're on requires massive amounts of machinery and power to run (but can be disabled by pulling a single cable ;) ), no functional machinery of any kind is viewable on the other side.

We know Combine portals must have two ends - Alyx says at the end of HL2 that Breen "turned over control [of his portal] to the other side", and when the portal does open, machines can be seen on the other end. Yet this portal seems to only need one end to be operational, meaning it could theoretically open anywhere, at any time - literally. So perhaps those images were a map of the other world, and they were trying to find the best locations to go to.

Oh, and as for it being Earth... unlikely. Minerva refers to it as "that planet", meaning it differs from the planet we are currently on. But maybe I'm reading too much into two words.