Jude
I am not an expert, but I thought I would reply while your waiting. One of the reasons you can't force Avast to uninstall is that it tries to protect itself from that, because this is what a virus would try to do. In the future if you want to try to do anything like this, go first to "settings" then "trouble shooting" and un-tick the "enable avast protection" box.
I had a problem something like this, and If Avast gets corrupted, it sometimes fiercely attempts to protect itself as though it was under a virus attack. Mine happened when I loaded an image in which Avast registration was out of date. The program was disabled, I couldn't register it, and it wouldn't uninstall. I downloaded the Avast uninstall program from their site and had to go into safe mode and that took it off.
While your waiting, maybe you could give it one more try. Download the avastclear uninstall program from their site, and run it in safe mode. then see if you can install fresh program and registration.