> There are thousands of similar but not equivalent ways of doing the same thing, both StackOverflow and the emacs wiki contains some

What about the manual?

https://www.gnu.org/software/emacs/manual/html_node/emacs/Di...

The issues you described are mostly totally solved by looking in the manual first rather than externally.

> Why oh why couldn't they improve linenum-mode instead of introducing a new mode.

The double-edged sword of being very backwards compatible.