Your thread title asks if the outputs are compatible but then you ask if the sources can be opened in the different versions.
WebHelp from RoboHelp 10 is likely to have issues with modern browsers.
As far as source is concerned, go ahead if you want to wreck your projects? The two are not compatible. You can upgrade 10 to 2017 but going backwards will create issues.
Multiscreen is not the same as responsive. You had to configure each screensize for multiscreen. Responsive does it for you. It is much easier and most people have long since stopped using mulitscreen.
It has been dropped in 2019 New UI.
Please use the blue Reply button at the top to help me help you. The black Reply link nests replies and they sort out of order.