W.r.t your hidpi/non-hidpi issue - a 'proper' solution is slowly making its way out of the woodwork. Wayland apparently will allow us to solve this problem, but in the meantime here's my solution if it'll help you.
I have a 4k dell xps 13 and a 2k monitor. I have this script set up to run on boot.
What this does is it forces your internal display to run at exactly quarter resolution (ie 1600x900), and adds your second display at its native resolution to the setup. Since the panel is running at a multiple of its native resolution, it still looks fine, if not quite as pixel perfect. This has the nice side effect of making the laptop feel a bit faster since it's driving a quarter of the pixels it usually has to drive. This makes the monitor panel do the scaling too, which is nice. With this setup, both displays will be running at a roughly similar DPI which will make scaling issues... a non issue!
I have a 4k dell xps 13 and a 2k monitor. I have this script set up to run on boot.
https://gist.github.com/kn100/7193667d393fcf34d4d5c6c359a695...
What this does is it forces your internal display to run at exactly quarter resolution (ie 1600x900), and adds your second display at its native resolution to the setup. Since the panel is running at a multiple of its native resolution, it still looks fine, if not quite as pixel perfect. This has the nice side effect of making the laptop feel a bit faster since it's driving a quarter of the pixels it usually has to drive. This makes the monitor panel do the scaling too, which is nice. With this setup, both displays will be running at a roughly similar DPI which will make scaling issues... a non issue!
I hope this helps.