Movie apps should work. I use them all the time. Like...all the time. So no idea what's going on there.
Games with online leaderboards track those for you. No need to to post them yourself. Most games let you post screenshots of leaderboards if you want though in the miiverse community, which your friends should see on their feed.
Posting in any community will show up in your friends feed so you can communicate with them that way. Or you can just send them a straightforward message. Unfortunately, messages don't come up as notices on screen while you're playing, only when you hit the home button. The home button flashes, but like anyone notices that. You can also video call friends if you want, though you might wait a minute or so for the connection to actually go through. What the system REALLY needs is a system wide party option.
I'm not really sure why you're having trouble with eshop. I use it constantly, have bought around 6 games online, and never once had it go buggy or crash on me. Sounds odd, not sure how to help you there.
Don't add money to your balance, just buy directly using your credit card. You can pay the exact amount and you wont ever have any space change in odd amounts hanging in your balance.
While just about any powered harddrive will work, some people have had problems with usb powered drives. They're supposed to work but it's apparently hit or miss. Nintendo does have an official list of drives that are guaranteed to work though. Definitely pick one up because buying games, downloading them, deleting them, and redownloading them when they're that big sounds just downright awful. The list can be found
here.
Sales aren't up to nintendo. 3rd parties have 100% control over them, which is why some games like Sonic and Tekken are a little cheaper. If you want to go all digital though, you're unfortunately going to have to buy a harddrive to store those games. It's an extra added cost and that sucks but such is life. I wouldn't expect anything like steam sales on the eshop though. The only games that seem to have regular sales are indie games...we'll see if any of the other publishers start discounting their games anytime soon.
Anywho, I hope some of that helped