Newbie needs basic advice on casting small hollow part

Yes a two part mold is what you need to make. I'd put the pour hole on that piece in the middle that sticks up and maybe a couple more along the top edges so that once the casting is poured, you can just snip off any excess.

Be sure to carefully clean all dust, dirt, and oils off the button before you go making the mold. A little soap on a rag or paper towel should work fine and be sure to dry it afterwards. When casting the mold, "brush on" some of the mold material onto the master (the button you're making a copy of) before pouring into the mold box so that you can prevent some bubbles from appearing on the surface of the master when you cast the copy.

I'd position the master upright with that slanted edge on the bottom. You can do that type of "injection" with a two part mold but I'd also make sure to "brush" on a thin layer of the casting material over the surface of the mold first.

Yes you should be able to mix the dyes to get the color you want, just be sure to mix them into Side A before mixing in Side B so that you have enough time to pour once you have the color you want.

No, the more questions you have the better your molding experience will go - we've all had to start somewhere and it's no problem answering any questions you may have.