Not sure what you are trying to do here. Imo you shouldn’t use SOAP UI to test rest end-points. It’s not that you can’t use it for this purpose, but it will be messy. Use postman if you are testing a rest service. If you are implementing a SOAP API, then use SOAP services, not rest resources.
This is not true, it does support post method.
By the way, what you are trying to do is not clear. Please clarify what you are trying to accomplish, not your attempted solution. Show a screenshots of your flows and your requirements if possible.
As @engin_arlak notes, need screen shots or the details about the FLOW services and components you created. Share as much as you can – folders, service names, properties panes of the components, etc. The steps within the FLOW service are not important at this point.