Plugins are different, and therefore whether they can handle single or multiple inputs really depend upon its configuration and design. The same goes for API endpoints. If you design an endpoint that can accept multiple inputs, then you should be able to send those inputs to the endpoint.
Re access to the actual files, that also depends on how you design your flow. But very likely the design will involve a managed folder pointing to your local FS where images are kept. Then DSS would read data from this folder and process according to your flow.