If you're comfortable with the command line, you can use the dpkg command to install a .deb package.
The format would be sudo dpkg -i name_of_package.deb
then, sudo apt-get install -f. the dpkg -i command may leaves the package status in an "unconfigured" state due to missing dependencies.
sudo apt-get install -f will try to fix it downloading and ...
There's a lot of content in this "question" so it's hard to fully understand what you're asking. It's often better to ask a succinct question if you want a succint answer, but I'll try and answer some of your points.
In summary, if you have forgotten your password, it's not just installing software you're going to struggle with. Changing certain ...
I finally got it fixed.
I had to remove the apt.llvm.org repository from the sources.list. Then I could use the Force Version command in Synaptic to switch over to using LLVM builds from the kisak PPA (which still provides i386 builds of LLVM). That enabled me to reinstall steam, because it could finally get those 32-bit i386 library builds again.
Welcome to the elementary OS StackExchange.
Then I tried to do it. Error after error.
It would be useful to include these errors in your question. No relevant help can be provided when no relevant information is supplied. Please read on How do I ask a good question.
old MacBook from 2009
There is a huge number of tutorials available online for ...