It seems like you are asking some JanusGraph-specific questions so you'll likely get better answers if you ask these in their Discussions board (
https://github.com/JanusGraph/janusgraph/discussions) or their Discord server. I'm not familiar with JanusGraph but I'll try to provide answers from a general TinkerPop perspective.
1. You only mentioned a .properties file which generally describes the Graph instance but isn't used for loading data into the Graph. The .yaml file usually contains a line like:
"org.apache.tinkerpop.gremlin.jsr223.ScriptFileGremlinPlugin: {files: [scripts/empty-sample.groovy]}"
which points to a script that actually loads the data.
2. I think JanusGraph has this functionality in its implementation of the server. See
https://docs.janusgraph.org/operations/configured-graph-factory/ for more details on setting up JanusGraph and
https://docs.janusgraph.org/operations/dynamic-graphs/ for how to use dynamic graphs.
Again, for more JanusGraph-specific help, I suggest you make a post on their discussion board, Discord server, or make a StackOverflow post and tag "janusgraph".