Yes, you can compile the source for yourself, or buy the plugin and get access to the binaries, and support. Keep in mind that buying the plugin supports the author of it!
Install java 8. Java 8 is required for this plugin to work, no Java 7 version is available! If you are using shared hosting, you'll have to contact your host support.
Remove the Banners item from the config.yml, and disable Banners in banners.yml
Not possible, I depend on a lot of Spigot features, and you should use it if possible.
Open a new issue here: https://github.com/j0ach1mmall3/UltimateCosmetics/issues/new and I'll see what I can do about it.
If you paid for the plugin, you will receive free support. If you didn't pay, you'll have to figure it out for yourself.
Open a new issue here: https://github.com/j0ach1mmall3/UltimateCosmetics/issues/new