Welcome to the forums @baznat!
1 - I generally use 1080p or 720p when developing, but most of the games I develop are desktop first, with a potential mobile port later.
2 - For any Control-based node, which is most of the UI, I use layouts and anchors so the positions of the elements stays at the correct position even if the screen is scaled. Then I generally set the aspect mode in the project settings to expand and the scale mode to 2D, which works decently.
3 - Not really, especially if you have the project scale the resolution down for devices that have smaller resolutions. In theory, it shouldn't matter too much what resolution you are using to develop if you have the scale setting to 2D, as it will increase/decrease the rendering size so it fits the resolution of the device that is running the game.
Disclaimer: I'm mostly a 3D game developer who targets desktop platforms, not mobile, so please take what I wrote above with a grain of salt!
