I think it doesn’t matter nowadays. Network interfaces are smart enough to twist them internally, or at least, this is what I experienced. I have no idea when did I had to use specifically a crosswire cable, all of my ethernet cables are patch cables for a while now.
So, it shouldn’t be a problem.
With Termux on Android, it’s possible to install Debian in a proot environment.
More about this here: https://github.com/termux/proot-distro
I guess using this envirinment, it’s possible to set up your server to your needs.
One thing I don’t know is how to autostart Termux at boot, but there must be a way, “worst-case” a Tasker routine can do it. Also, another thing to considerate with this setup is Android’s memory management and to detect if your proot distro gets killed. But then again, maybe Tasker is good for that, too.
Good thing with this proot-distro thing is that it’s really easy and straightforward to set up, and basically you have a full Linux environment in your pocket. There are limitations, of course.
As for installing/flashing full Debian on an Android phone, let’s say it’s not possible. Your closest best bet would be Ubuntu Touch on a supported device.