Looking back on this thread, here is what I think:
1) For some reason, your system is not correctly detecting your built-in trackpad - hence it is asking you to connect a bluetooth one.
It is strange, seeing that it works and all.
Two possibilities for this:
a) some weird hardware failure;
b) some weird software conflict.
At this point, my "money" is on b); which takes me to...
2) Reinstalling over the existing system is not going to help.
Doing a clean install probably will. But before you commit to doing this, you can try installing Mojave on an external drive and testing.
Before anything else: please run the diagnostic tool Etrecheck and post its report here.
It is the best way for us trying to help to know a bit about your system.