First of all, in Linux everyone should only use software from distribution repositories (eg. via apt command in Debian, Ubuntu, Mint, dnf/yum command in Fedora etc…). Package managers will install software in controlled way and it is really easy to remove them too. And, there is usually gui app for installing apps from distribution repositories.

Second way is to use flatpak / snap. They are pretty much similar and will keep things easy.

Do not install sh packages or tar.gz if you really do not know what you are doing. These are only for expert cases.

One fundamental change coming from Windows is that in Linux, you should never worry about location where software is installed (except for those expert cases, which you should not use). They will be put in correct places always. In Linux, apps are sorted so that executables go to /usr/bin, library files to /usr/lib64 and /usr/lib, applicatoin other non-modifiable stuff to /usr/share etc. It gets quite a lot to get used to, but in long term it feels more natural than Windows way to dump everything in app directory.

My recommendation will be to install some user friendly distribution (Ubuntu, Fedora, Mint) and just go ahead with default package management things what it offers. If you see Android way handling software good, Fedora Silverblue is kind of like that - System upgrades are handled same way, and applications are installed as flatpaks.

Can you access your wan ip when you are somewhere else than on your own lan?

If not, then this is probably just that your router does firewalling and nat is such order that you can access admin interface from local network via wan address.

If yes, then router has some serious misconfiguration.

Blog makes valid point, but why on earth there would be any current Linux distribution without usr merge?

EDIT: Especially when every major Linux distributions have already implemented usr merge long time ago.