rj_sosongco speaks the truth. Adding to what he said:
3: I haven't used silicone caulk as a mold personally, but as long as you let it cure, it should work fine for all casting resins. The mold must be strong enough to not wobble or deform. If there is a chance of this happening, you want to make a rigid mother mold.
You can use a rigid mold (such as plaster of paris) for casting a rigid piece if the piece has no undercuts. An undercut is a feature that once cast would cause the rigid piece to lock tight into the mold. Undercuts are safe when either the casting or the mold is flexible (within reason) because it'll just peel out. Another way to get around undercuts is to design a multi-piece mold. However, the more pieces in your mold, the more complicated it becomes, the more places for you to screw up, and the more seam-lines that need to be trimmed away.
If you make a plaster mold, make sure to use a release agent. I like to use a commercial mold release, but you can use PAM cooking spray in a pinch. Give it a minute to dry/soak in and then dab up any excess.
4. Polyester resins all have fumes. Epoxy has fumes, but they are less noxious, and they are less detectable after the piece has cured. Polyurethane resin has almost no odor, though it should still be used in a ventilated area.
Paper mache clay:
Paper mache clay should really only be used to repair flaws or add complicated details. The base should be paper mache. If you do paper mache carefully and with the right paper, it can be nice and strong. Waiting for it to dry is the only really annoying bit.
1. You can speed the flashing off of the fumes by building an oven box. This is basically a cardboard box lined with aluminum foil with an incandescent bulb or sun-lamp in place. poke one hole in the bottom of the box and one hole in the top to allow just enough airflow. Then let it sit in there for a couple days. You have to design this properly or it is a fire hazard. Barring that, yeah, outside in the sun for a few days.
2. When the fumes are almost gone, yes, a coat of Killz paint helps mask the odor.
3. Just how harmful these fumes are is under debate. This is partially because it is hard to scientifically prove something like that one way or the other. After the piece has had a few days to cure, it is generally considered that wearing a resin mask a few days a year won't seriously harm you. If you feel dizzy or anything, don't be a hero; take a break.