B2B has to write the zip file to a server location, unzip the file using a callout, check for the existence of .xml file and if .xml file exist call a URL using HTTP POST.Why you want to do all this at B2B layer? I think you should pass the zip file to the middleware and perform all processing there.
1) Is it possible for Oracle B2B to accept the zip file over AS2 and identify the sender?Yes, please refer -
2) Can B2B unzip the file using callout? If so, which callout is preferred i.e. the callout at agreement level or the callout at channel level (transport callout)?Yes, you may use agreement level java callout here.
3) Can B2B call a URL using HTTP POST if the zip file has the specific .xml file?B2B won't do anything automatically. Either you post the message over HTTP using same java callout or create a new trading partner in B2B and push a message for that trading partner when XML file is found in the zip.