Here are your answers..
1) Use this plugin https://wordpress.org/plugins/wp-maintenance-mode/
2) Make sure you have selected a menu position (look under your menu items in the menu edit screen)
3) Use this plugin http://www.woothemes.com/woosidebars/
4) Here’s the twitter one https://blog.twitter.com/2015/twitter-plugin-wordpress. And this looks like a good one for instagram https://en-gb.wordpress.org/plugins/instagram-feed/
5) Canvas is a commercial theme available from http://www.woothemes.com. We are also close to releasing our own highly customizable theme. This should ready in the next week :)