it's just a one-off adventure with no consequences for or hints about the show's plot
So... It's like a ton of other toku movies?
Honestly, I thought Magic Land was a pretty good Rider movie, all things considered. I had a few complaints, but they were minor and more movie-specific - it was enjoyable for the most part. Hell, I'd say the fact that it was a one off thing, meaning it had to keep its plot all together and concise rather than dragging it out like the show, was something in its favor, if anything.
1) If everyone in Magicland is a mage, wouldn't that mean they all have Phantoms inside them?
2) Did Sorcerer just conjure a world out of nothing, with duplicates of people in the "real" world, just to create Phantoms out of them? How can you make Phantoms out of people who aren't real?
Both of these things were addressed in the movie. They do have Phantoms, and that world IS the real world. It replaced the old one. They're as real as any Gate in the real world was.
3) If the Magical Exchange system is supposed to store magic in a giant vessel, what's the point of kidnapping people and using their skeletons for that vessel? You're getting magic from them anyway, aren't you?
I'm pretty sure this was explained too, but I believe it was to ultimately do what he did at the end. A slow drain where it's used as currency isn't going to jolt them into Phantoms like he did at the end there.
4) Why are the only Mage Riders we see the "earth" type? Where are the water and hurricane Mages? In fact, why isn't there a fire Mage? The closest we see is that captain with the red cloak, but even he has the same orange mask as the others.
I don't think they're actually tied to elements (since we've seen them use all the elements throughout the show regardless of their color), I'm pretty sure the other two just got different colors so they looked different from Mayu. Haruto is supposed to be the "fire," since if you line him in his default form up with the show's three Mages, you have one Rider of each element.
That said, here's an actual reason for why Haruto might be different: He survived the ritual, whereas they're just regular Gates that overcame despair as it came somewhat normally. He might have gotten a supercharge through experiencing that and making it out of it, whereas a normal person might not.
Then what's the point of having blue and green Mages in the show? Was this a case of two different ideas being thrown together at the last moment?
They never once said the Mages were a specific element, and nothing about them implies it. If anything, the default Mage is just "generic magic" color, since it's the same color gem as Wiseman/White Wizard and the basic Wizard Rings all are.
They don't make an issue of it, if that's so. In a world where everyone studies and uses magic, that would mean they or someone else would've had to conduct regular Sabbaths, or Phantoms are bringing people to despair on a regular basis. Wouldn't people object to that?
What if Phantoms just escape from people normally? For example, if a person who just happens to be a Gate experiences despair on their own (something that is perfectly possible without Phantom interference), then their Phantom would be born regardless of whether or not another Phantom helped push it along. The show never explicitly did anything with it, but considering they existed outside of rituals, that always seemed understood to me - that they're a natural phenomenon regardless of what rituals happen or don't happen.
But what's the point if he just wants to create more Phantoms? The mages in his world, since they're really just from "our" world, would have to have Phantoms in them already, so he could just do a Sabbath on the next solar eclipse. Plus Shunpei and Rinko had had their Phantoms killed already, so how can they have them again?
I'm not sure what you're saying in the first part, but Rinko and Shunpei don't have them "again." They never lost them in this world to begin with.
