a) It is possible but often it is adviced to be cautious. Search the forum and the wiki for some hints/tips and also read the docs. The impac totally depends on the processdefinition, how complex it is, what states running processes are in etc...
b) not at the moment afaik
c) What do you want to know? complete usecases of big companies?
d) No, but maybe by using jboss esb it will be fairly easy to do