Note: Extreme Launcher doesn't ship with any of the files required for Minecraft, you need an internet connection in order for the launcher to be able to download the required files from the official Minecraft servers. After the initial download you can use Extreme launcher without internet connection.
Try redownloading and reinstalling Extreme Launcher, it's recommended you use latest version.
If you're getting an "Invalid session" or "Disconnected" error you're trying to log in to an online-mode Minecraft server which requires paid Minecraft account, while you're using an offline-mode account, there's no way around that.
If you're trying to log in to an offline-mode server and you see this error, then you could try changing your Minecraft username.
No, Extreme Launcher is Free, no registration or account is required to use Extreme Launcher.
Skins are tied to Minecraft accounts, offline mode accounts will have default skins. On some servers, skins can be changed using in-game commands.
Yes, mods like OptiFine, Forge, etc. are fully supported.