I don't think that Demi-gods can manifest on the material plane, just heroes.
And yeah, it might not have been specified but the prime-material plane is basically another term for the starting plane. It comes from setting where all the planes are already established and refers to the plane on which most adventures happen, so it has all the races, philosophies, and elements coexisting, most other planes are specifically attuned to something present in the prime material plane, such as Evil or Air or the force that animates undead.
The plane of shadow is a bit of an anomaly, it is sort of an overly complicated reflection of the prime material plane, I think that stuff that happens on one can kind of influence what happens on the other(mostly the prime material affecting the shadow). But basically it is just a cheap excuse to have 'through the looking glass' adventures where everything is familiar but different, and kind of sinister...
[OOC] I want to use it as kind of a warehouse, storing all the cool stuff I collect.[/OOC]
*I am influencing the scribbles at the south pole for purely RP purposes and don't plan on doing anything more with it, if I do I will spend AP to do so. If a god manifesting nearby in an adjacent plane doesn't have that much influence then I will gladly remove it.
Well, I guess he'll have to find something to gamble with. Not sure what there is to work with. Since I can't create planes, I have to mess around with other gods' stuff, right?
As a hero god you should be able to walk around the place beating up scribbles and stuff without going to any effort at all, you can probably even build a city or something and pave the roads with this shiny new metal that is getting dumped all over the place...
I could probably be persuaded to lend you some AP if you pay them back, but I don't think that my god has noticed you yet...