Heyo! Iggly also has Projection and Reimancy. Projected limbs can in fact be used as origin points of Reimancy. To people without the ability to see the limb, it'll look like the Res is seeping out of thin air.
I'm not sure about question #2, but as the astral body seems to count as a part of the body for the intents of Reimancy, I would lean towards yes. Very creative.

Question #3: Yes. It's ethereal, the only thing that would stop a Projected limb would be a Shield or other magic-blocker. Maybe a highly skilled Spiritist could set up a safe zone that would apply to astral bodies. A Magecrafter could probably make a projection-proof door or something. However, astral bodies will go through rocks, living bodies, frogs in jars, giant marshmallows, anything. Res will get left on the other side of a non-porous object, but it
can be produced once the Projected body part is already inside the thing.