[I'm being polite :)] I've been unable to find anything to make an informed opinion on what they mean by end-to-end encryption, which is prominently mentioned on the home page etc.

[I'm not being polite. #BeingThatGuy :)] My best guess is that they're using their own definition of end-to-end encryption. i.e., SSL for transit + encryption at rest = "end-to-end encryption". Whatever.

If you forget your password, you lose your files. There is a 'forgot password' option which you can disable when signing up.

https://www.sync.com/your-privacy/

Figure out what you want from the page.