Tell us about yourself!
Source of the skulls? I guess watch for them in dollar stores around Hallowe'en...
Not true at all. I've had 3V 3032 cells last for several weeks with a 20mA LED.
Not really. The button cell is only 3V and will not affect the LED.
Yes. The LED will be just a tiny bit dimmer, hardly noticeable.
We call them BLFNARs (pronounced "Blif-nars"), which is Blinky Lights For No Apparent Reason. A vital part of any electronics project!