Hi Ako,
Find my answers below:
@1: Yes, you can call existing Web Service from your ECC by creating Entersprise Service (Service Consumer) without any additional middleware (PI/PO, BizTalk, etc.).
@2: I do not really know what do you mean by that. Using middleware you can map message and route it to various recipients. If you call WS from ABAP you must fill in message content using ABAP code.
@3: There is no need for having RFC (unless you want to call it via RFC). If you already created Enterprise Service (Service Consumer) and you have specified External WSDL/Schema option and provided WSDL along the way that should be fine. Then you have to create logical port in SOAMANAGER (which you did). Please remember to activate that port and most preferably set it as default. Try testing your Enterprise Service directly from SE80 - there you have an option to specify which port to use - your port should be available in search help result.
If you have any further questions please let us know.