I will try to answer your questions.
1.You could use jBPM on any scenario, like J2SE, servlet container tomcat, jetty, JEE container like jboss and etc. The database is necessary for using jBPM because it have to persist all of data to the database.
2.I am not very cleary for your question. Do you mean you want to start process from any activity without knowing the process definition? If there is no relationship between them, I am afraid it is impossible.
3.If you want to let process instance jump to another process definition, you could use use-process activity. You can't let a transition from a process definition to another.
wrong forum - Please use parent forum for such topics - this forum is intended for development of JBPM code base