The discrepancy appears to come from the installer looking for nemo-python and python-nemo respectively. The backported package has the name reversed.
Installing the nemo-python or python-nemo will suggest that each will break the other.
So, you MAY have nemo-python 4.40 installed. If so, you must remove it. You can use the Self-Installing .deb package to Remove instead of install. Here is the link to that package, if you need it in order to use it to uninstall/remove it:
http://archive.ubuntu.com/ubuntu/pool/universe/n/nemo-python/nemo-python_4.4.0-3_amd64.deb
Install only the python-nemo_5.0.0+uma_amd64.deb package:
Once that is installed,
Here is the Nemo Media Columns installer:
https://ftp5.gwdg.de/pub/linux/debian/mint/packages/pool/backport/n/nemo-media-columns/nemo-media-columns_5.0.1+uma_all.deb
Once installed, launch Nemo File Manager and click File > Preferences
Go to Plugins > Extensions and ensure that Nemo Media Columns is checked to On.