Should this still allow the possibility of browsing both directions?

Say we have a --direction=forward|backward|both flag with forward being the default option.

Nope, if loaded with only one direction, traversals will only be possible in the loaded direction.
This will essentially be a trade-off setting for people who cannot (or doesn't want to) load both direction.
It is fine to fail (gracefully, with an error) traversals requested in a direction that corresponds to a non-loaded graph.

The default loading behavior should either be "both" (overridable "down" to a single direction), or alternatively "whatever is available on disk".

Copying my comment from a linked diff:

The design I have in mind is to split the SwhGraph into two different layers:

• a SwhGraph interface that will contain the swh-specific metadata handling (node types, SWHIDs etc)
• a BidirectionalGraph interface on top of ImmutableGraph that will take a graph and its transposed version, and define indegree(), predecessors() and transpose().

Using this, we could:

• Have a generic bidirectional graph by using a SwhGraph(BidirectionalGraph(graph, graph_transposed))
• Have a swh-specific unidirectional graph, either forward or backward, by using a SwhGraph(graph) or a SwhGraph(graph_transposed)
• Have a swh-specific bidirectional graph by using a SwhGraph(BidirectionalGraph(graph, graph_transposed))

Note: I don't know yet what is the correct data model to implement this in Java as I'm not too proficient with interfaces/mixins.