A few considerations.
It may be possible to return to using the nouveau driver without purging the nvidia software, but the problem with not purging the nvidia software is that if the system still has a problem after trying to disable the nvidia software to run nouveau, then the user has the problem of determining whether it's the remnants of the nvidia software that are interfering with the graphics, or something else. It's potentially a situation of having a number of stray variables from the nvidia software interfering from being lodged in numerous locations of the filesystem.
In this case, it looks to me that this scenario I have described has obtained in your case. It's the nvidia software that has blacklisted the nouveau driver in /etc/modprobe.d/, and this has not been removed or purged, so the system is getting a double message which is to both blacklist and to run nouveau at the same time.
It would have been more helpful to the system in this case to purge the nvidia software.
Here is a summary of purging from the link in post #10, for your consideration, though not all commands may be needed:
====== summary start ======
Run:
Libraries can be missed with purge, so:
Run: dpkg -l | grep -i nvidia, to see what's still there.
Run: apt purge nvidia-* , to purge.
Run: apt purge nvidia-driver-*, to purge.
Remove dependencies:
Run apt autoclean
Run: apt autoremove
Run dpkg again to check again
dpkg -i | nvidia
Do a cache search in /home/<user>, and elsewhere, /var/cache, and delete nvidia remnants.
====== summary end ======
From the output in post #12, it looks like the system is running nouveau, but from your reports, not properly.
The nouveau driver should be removed from the blacklist if it's going to run properly. It was the nvidia software that blacklisted it. The nouveau driver needs modesetting, so the modeset config is inappropriate and should be deleted along with the removal of the blacklisting.