You don’t need reproducible builds. You can get by if you trust whoever compiled it, like your distro’s maintainers or the pidgin developers.

Copyleft means: “if you modify the program and share it, you also have to include the source code for your modifications.”

The owner of the copyright (usually the developers or their employer) can still change the license later.

The real interesting debate is between ((f) 1) and f()(1).

In the sense that a CD player learns new music when you insert a disc, yes.