Hi Ray,
In short, both questions are YES.
1. You can call web service, make a POST request with XML content and trigger the process. If that format do not work, you can add Composite API in front that allows much simpler input.
For the XML case. In process design look for "Open corresponding connector"button. This gives you the details about the WS call you need to make.
Authentication scheme is simply, standard Basic (id:pass base64 encoded).
This guide might help a little (not much, sorry)
2. You are right that when you use activity type Subprocess, the main process will wait for it to complete.
You can use activity type "connector" and make the same API call as I explained in above answer, then main process will just move forward after initiating the WS call.
Best regards,
Taka