Hi JB,
with all the other things you mention that aren't working like the arrow keys for example, leads me to believe that you may have burnt your Ubuntu Mate disk too quickly if you used a disk to do your install!:
If you did a USB install, did you fully format the stick to FAT32?.
The only other option that I have for you is to try booting into recovery mode again and activate network; this time drop to "root" and type the following commands:
sudo dpkg --configure -a
and then:
sudo apt-get install -f
as I suspect that you basically have a damaged install and it may be possible to fix it with those commands!.
Once done, restart with Ctrl + Alt + Del, do not click on resume!.
Rgards Wolfman 