I would start by browsing the UI & Macros forum at the site in the related links. It is on the official wow forums and will give you a huge head-start in your addon development.
Depends on what addon you want to make...
You need to put the addon in to your addon folder. The easiest way to do this is download the curse client (free) and then anytime you want a new addon just install it via the "Install VIA Curse Client" button.
No, most addons will not get you banned. The only things that addons are allowed to do in game are approved by Blizzard. If Blizzard does not want an addon to do something, they just make it so that addon can not do it.
Button forge is an addon, you would be better off going to the addon authors help site if he has one, or the wow forums. They will be better able to help you.
use an addon by the name of "https everywhere".
download the addon. Should be on a zip file. Simply extract the file. place the folder with all the files in your >world of warcraft>interface>addons folder.
There is an addon by Creative Tools called "My Chrome Theme". Download this and you can customize from there.
You need to have addon
Here is the normal installation path for the interface/addon folder: C:\Program Files\World of Warcraft\Interface\Addon
the only jeapordy addon found is linked below to the Curse addon site. It adds some sounds for certain events.
GET FIREFOX, (IF YOU DONT HAVE IT) DOWNLOAD THE ADDON ILLIMITUX AT https://addons.mozilla.org/en-US/firefox/addon/11037
You need to put the addon in to your addon folder. The easiest way to do this is download the curse client (free) and then anytime you want a new addon just install it via the "Install VIA Curse Client" button