Hyphenation: yes, justif does its own hyphenation. It ships with TeX’s hyphenation dictionaries for a bunch of languages, but supports custom dictionaries as well! See https://github.com/lyallcooper/justif#hyphenation

Narrow columns: open up the controls on the demo and try adjusting the width slider :)

Long documents: try the “Alice in Wonderland (long)” sample text in the demo to get a feel for how it handles long documents. In general it performs well enough I think, though Safari is a little slower than other platforms. It doesn’t do it incrementally (not sure that’s possible), but I spent quite a while trying to make sure it was fast enough to re-layout in real time even on long documents.

I haven’t thought about coexisting. I’m not sure how that could work but I’m open to ideas.