#2: The techies will tell you, but very few things are totally secure, from what I have read/seen.
As far as online storage services are considered, "very few" may as well be "none." Dropbox has been hacked repeatedly. As has Google Drive, OneDrive, iCloud, Amazon S3, and a lot of other services most people have never even heard of. But there's good news here: hackers like to target places like Dropbox because actually getting into your Dropbox account is a secondary benefit. The primary thing they want is your username and password. That's because most people use the same login and password for
everything. So, once they can get into your Dropbox, they're in your bank account too. That secondary benefit is the off-chance that you might have something in your Dropbox which could be valuable to them: anything from private photos to personal information. But what writers would use Dropbox for is of little interest to them. They generally don't care about your WIP.
The good news is that you can mitigate this problem with password storage services such as LastPass (paid service) and KeePass (open-source free service). The idea is that you can keep your one simple phrase-based password in order to open your password database, and then let the program create incredibly complex passwords for you. 24 totally random characters is more difficult to crack than passwords based on phrases (yes, even with number/letter substitutions). It's like having a keychain in your pocket. It's just the tiniest bit of a pain in the ass when you have to sign in from a device you don't own, and therefore must painstakingly copy your password letter by letter from your database. The bonus is that this way, you can easily keep different login/password pairs for every site that has any of your sensitive personal information (SSN, DL#, payment types, etc), and then change them regularly (you should do this).
Look at it like it was your keychain. You wouldn't want the same key that opens your car to also open the front door to your house, your safety deposit box, or your home safe. You want your keys to be complicated and hard to duplicate. And you want to be able to occasionally change the locks, especially if you're in a bad neighborhood.
Keep in mind that for places like these wonderful forums aren't something that you have to engage this super-high-paranoid mode for. But confidential work information or things that could help attackers commit credit card or identity fraud definitely deserve the best security you can manage.