Random thoughts (and therefore probably a bit disorganized):
Suppose you are wandering the Ethereal Plane (or Astral as appropriate) how visible is a demiplane (from the outside looking in so to speak)?
Is it invisible from the Prime Material but visible from whichever plane you created it off of (either the Ethereal or Astral) where you see whatever the boundary you created via the spell exists (i.e. The “walls” and “ceiling” of the plane may appear like solid earth, stone, wood, or water, or they may end in mist, a featureless void, or a similar unreal-looking border)?
Seems to me once you are at the boundary while either Ethereal or Astral all you have to do is 'step over' the boundary to enter the Demiplane. Or how else would those travel spells actually work to enter the Demiplane? Therefore yes you would have to worry about coming home to an invasion party of Xill, Phase Spiders, Githyanki or 'name your GM's favorite Ethereal or Astral foe.' This is probably one strong advantage of using Mage's Magnificent Mansion over Create Demiplane. Sounds like a really good idea to make a real barrier around the perimeter of the Demiplane if this region of the Ethereal or Astral is more active than say western North Dakota. Gets more optional the less paranoid one is and the lower the adjoining population density is. Finding the boundary that correlates if you don't know what fixed point is relative to both is another matter.
The following spells work to enter the Demiplane:
Astral Projection (RAW)
Plane Shift (though there's the rather rule contradictory randomness involved) (RAW)
Create Demiplane all 3 versions (maybe)
Shadow Walk (maybe and probably only if off the Ethereal)
Wish and Miracle (RAW)
Anything else I'm missing?
The list to exit is, if anything, shorter as it doesn't by RAW include the Create Demiplane spells. And Astral Projection might be both weirder and better than most of the methods as your 'second' body is now the one wandering about the Material or where ever. Getting ejected works too but this whole thread is about what happens or if you can do that :p (and it's not a separate spell effect either).
The 1 minute/level duration on Etherealness leads me to conclude the location of the demiplane must remain fixed relative to the Material, Astral and the Ethereal or the usefulness of Etherealness to enter the demiplane plummets for the caster or other repeat visitors (and the spell Create Demiplane seems to imply it is useful).
"You are considered “very familiar” with your entire demiplane." And the relevance of this portion of text is? Did they really put this in there just to let us know you could Teleport around inside your Demiplane without going around studying every square inch or is there something more profound I'm missing?