1.) Spirit gum is designed to be safely applied to skin. If you know you have super sensitive skin and are concerned, get some and do a spot test on your skin beforehand to see how your skin will react. This is something only you can determine because most people can use spirit gum with no issues.
2.) Depends on what you are applying and how much spirit gum you apply. For heavier appliances like large elf ears or large facial prosthetics, your wear time may be diminished or you may want a different kind of adhesive for the job such as Pros-Aide. Small appliances like sticking small jewels to your skin or applying small elf ears or a small scar appliance will require less spirit gum and will not be subject to the same amount of stress/wear-and-tear. If applied properly to clean skin and with the appropriate amount of spirit gum, things should stay in place all day. Do a test before the event/con you want to attend and adjust your plan accordingly.
3.) Re: spirit gum is designed for skin so it is generally safe. Unless your appliance is really heavy and bothersome to your skin or if you have super sensitive skin, you should be fine. Again, run some tests at home before attempting this at an event/con.
4.) Spirit gum remover will make the process much easier and will help keep the appliance (or whatever you are sticking to your skin) clean and read for its next use. Alcohol will work as will something oil-based such as baby oil or oil-based makeup remover. However, check that either alcohol or oil will not harm the appliance before using either of these. For the process: here is a quick reference link: http://www.ehow.com/how_5087740_remove-spirit-gum.html