Thanks for your questions! Can’t wait for you to get your JingPad either! As for your questions, I will give you a brief idea.
1. Which display server is being used?
We use Wayland as our display server
2. Can another desktop envrionment be installed?
JingPad A1 is based on ARM, so you have to check if other DEs support ARM or not.
However, we will open-source the code of JingOS for JingPad. If you wish, you could port other DEs to JingPad A1.
3. Which Desktop manager?
For desktop manager, we use kwin.
4. Will it be possible to install other distros?
We will provide an official Android ROM before mass shipping. Meanwhile, we will also provide help to the community port other distros, like Sailfish and Ubuntu Touch.