I use resilio sync (formerly bittorrent sync) for this. It provides bonus features (2) and (4), and with some finagling you can also get bonus point (3). It d

I use resilio sync (formerly bittorrent sync) for this. It provides bonus features (2) and (4), and with some finagling you can also get bonus point (3). It doesn't provide (1). https://www.resilio.com/ https://www.resilio.com/ Beware that you have to dodge a lot of premium functionality in the UI, and search a bit to find the secret key functionality. They originally built the app to work decentralized with just secret keys, and no accounts, but then they wanted to build a business out of it, so they started trying to compete with Dropbox, and they made a centralized service to wrap the decentralized functionality. Ignore that part.