Hi
do we use a BAPI/FM from ECC and create the webservice directly in PI?
yes we use bapi/fm or do coding in proxy and create the webservice directly in SAP PI.
http://scn.sap.com/thread/1804588
https://scn.sap.com/thread/1620037
https://scn.sap.com/thread/1616693
for mobile and sharepoint u can do soap ->pi->sap(rfc,proxy).
Thanks,
Dipak Patil