when using :git/deps does :deps/root need to specify a directory containing a deps.edn file or can it be a path to a deps.edn file named something arbitrary, like foo.edn?

I’m guessing it must be a directory

Yeah, directory

I’m assuming it’s not possible to override just the :deps/root for a :git/dep in aliases? Use case is sharing a single :git/url and :sha across multiple aliases that parameterise just the :deps/root. I’m guessing that to do this I would need to duplicate the :sha and :git/url as :extra-deps in each alias.

Yeah, duplication

I'd write a tool to manage it for me

Is there a trick to not need to repeat the version number here:

{:deps {org.apache.lucene/lucene-core   {:mvn/version "8.1.1"}
        org.apache.lucene/lucene-queryparser   {:mvn/version "8.1.1"}
        org.apache.lucene/lucene-analyzers-common {:mvn/version "8.1.1"}}

Nope.

Unless there's a bundled artifact you could depend on that would bring in all those pieces.