Is Pathways into Darkness (Aleph One) free?

Yes. Pathways into Darkness (Aleph One) is released under Freeware / Community and costs nothing. Neither it nor alephone has any paid content.

What do I need to run Pathways into Darkness (Aleph One)?

alephone, which is itself free. Pathways into Darkness (Aleph One) is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from alephone's mods list. You can also install it by hand — full steps are on this page.

Roughly 60 MB, on top of the base alephone installation.

Yes. The source is published at github.com/Aleph-One-Marathon/pathways-into-darkness under Freeware / Community.

Pathways into Darkness (Aleph One) — Free alephone Mod · PlayBound