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Reconciling Urth and the Hollow Worldby Kheldren
For those who don't have the old Gold-box Immortals set Urth is a Megalith. Megaliths are intelligent planets that have active and dormant cycles. Approaching one in a dormant phase is a bit of a mistake - they shoot magma at you.
Anyway Urth is the planet on which the Known World sits (note this is before the name Mystara was coined). In fact the theory goes that it is the same world we live on (which is probably why they set the Nucleus of the Spheres up to drain magic, but that is another thread).
Between the publishing of the gold box and WotI the Hollow World was invented, which put a huge hole in Urth (literally) and Megaliths were dropped from the rules.
How can Megaliths and Urth be re-introduced without destroying the Hollow World?
Urth, being a bit bored and immortal found a nearby plane that was nearly totally empty and created doorways to it within his crust. (He may have needed mobile helpers to find it and do some shaping, but Megaliths are on good terms with Draeden if I recall correctly.)
So Urth set up the plane to be a 'hollow world' with gates in and out from points in his crust. When people dig too deep he drops a gate in front of them (whichever world they are on) linking them between the worlds. For fun he also added the two massive gates near his poles (I would have Urth doing those not the Immortals since he probably needed to do some healing after the Blackmoor explosions - these gates prevent access to the worlds nicely).
The Immortals, seeing what Urth was doing decided to help - adding the central star etc and using it to dump lost cultures. I would think Urth originally planned to see what would happen naturally, but has no objection to watching the immortals play instead.
The fact that it is a different plane is what blocks transport in and out (no need for a world shield). Also since this would be an outer plane, it is reasonable for it's only planar boundary be to the Prime Plane - right within Urth. It is a pentaspace so magic etc. work, but perhaps an unusual one hence the lack of teleport etc. (unless you add them back in your campaign).
The Spell of Preservation is still there - put in by the immortals for the same reasons.
One thing I know I have not addressed is the burrowers and how they fit with the limits here, but I should think they would work OK.