Sync via Dropbox: Do I need a Dropbox App on all devices?

What do you mean? It's nowhere written that a Dropbox client is required, thus it isn't.
So shall we list all non-requirements? Where will this end. You do not need a steak knife to use Joplin, nor a screwdriver. :wink:

The other stuff (selective sync) has been mentioned in the forum a few times. Here's one: