Hi Allessia,
The answer for your three questions is YES! :)
1) You can use Drools / jBPM5 server for that, that expose REST/SOAP endpoint to interact with the engine.
2) yes totally, you can mix complex event processing or simple events to stimulate the process status or a group of processes
3) You can use jBPM5 inside your private application/services because it's licensed using Apache Software License v2.
Cheers!