But that assumes that these alien "bugs" work like ours. [Edit: Ninja'd!]The Rahi ones definitely don't -- they're biomechanical. The bug shapes are trivial; they work essentially the same as the biomechanical Muaka or Tarakava, just in a different body format. I can think of at least one giant SM bug, but then who knows how the biology of that actually works? And sci-fi is no stranger to giant bugs even taking place on Earth.
Personally I think it's clear that both the surface gravity and atmospheric pressure are the same as Earth.
Also note, it's confirmed the Great Beings did not make the planet. There's shaky evidence that "Greater than Great" beings did, though that could just mean the EL's fault for the Shattering. (I've covered this in detail in past posts.) I theorize that in super ancient times, some unidentified species we probably have never met (who I call Original Beings) seeded many Earth-sized worlds across at least one galaxy with EP, and it transformed them into megaplanets with all the gravity itself and may also have made the beings.
I also disagree about the oxygen ratio. You're assuming way too much based just on one piece of evidence IMO. Who is to say bugs there breathe in the same way ours do? And there may be problematic side effects with a significantly different air ratio. That said, I don't think it's been confirmed exactly what the air is, only that it's normal matter and no protodermis. Greg may have said "air is like ours" at some point though. (And this could matter to the atmo height, of course.)
I personally think there is a noticeable magnetosphere, yes, though exactly what mechanism would cause it with such a radically different core I dunno.
There is one thing that might work -- if it was essentially like Earth before an EP transformation and enlarging (powered by, I guess, the EP itself... though the mass increase there gets questionable), then magnetic field change resistance could come into play. Magnetic rocks could solidify in the same alignments as the transformation happened, because fields want to stay the same, and generate electric flow as part of that process. The EP might also ensure this works, and amplify and permanentize the field. So, it could in short be generated partially naturally, partially by a power in the same rock that absorbs gravity (yet another possible use for the converted extra gravity energy).
Or it might be purely an EP power or purely natural, who knows?
I think the Story Squad actually got a proposal once that was vaguely along these lines although to be honest I barely remember it. But I noticed in relation to that that the particulate from the Shattering that still orbits the BaraM/AM/BotaM subsystem is "pinched" in at two sides in a way that could be explained by a strongly out-of-whack magnetic pole alignment (plus some spin-related spiral arm effects like galactic arms). Of course the real-world reason for that is they wanted it to match the previously established UDD symbol. But it could be evidence of a magnetic field (we could even say the image's alignment was simply non-canon and the pinches are actually over the poles, but that's also shaky).
Here's that pic:
I suppose, just thinking of it now, perhaps twisted magnetic fields due to the moons might also explain the shape. Strictly gravitationally, though, it's difficult to explain, so I do think magnetics is the best answer.
Edit: Re: one island -- I presume you mean Mata Nui. Actually it's mostly easy to explain geographically, something I noticed way back in 2001. The only assumption that's needed is a prevailing wind from the south-southwest.
This is due to the island's shape, with a massive cliff (seen in MOL, and implied in other media) between the low-elevation Le-Wahi (which being first hit by prevailing winds would get most of the rain, explaining jungle) and high-elevation Ko-Wahi (explaining the cold).
Ta-Wahi (which was previously forested before the fire that made the Charred Forest) gets some rain, and Ga-Wahi as well, because that direction of wind still mostly hits them. The lava flow of course is explained by the lower elevation land on the east allowing the liquid lava to head that way, though south could have worked too (to solve this we can just presume that initial EP-caused landforms happened to channel it east and these might not be obvious anymore as the lava has stacked atop this). But neither get as much rain as Le due to those clouds tending to rain out somewhat first in the south.
And Po-Wahi is explained by the mountain shadow effect -- the wind is redirected and loops due to multiple mountains, so most moisture either snows out in Ko-Wahi or is deflected to the east and west, missing Po-Wahi entirely.