Q1: Yes, the REST API allows you to create users directly (see https://developers.jivesoftware.com/api/v3/cloud/rest/PersonService.html#createPerson(PersonEntity, Boolean, String, String) ) . Although if you are setting up SSO, a more common why is to let the SSO setup in Jive take care of user creation.
Q2: What do you mean by "launch from within them"? If you mean embedding Jive in an iframe of something similar, then, yes, this is possible, but requires a small bit of customization to remove the iframe breakout code (I think it is in template.ftl)
However, this is an uncommon scenario, and the UX is not great. Usually sites have a link named "community" or similar, that will take the user to the Jive system. In my opinion, Jive works best standalone, and not embedded or within a portal. Compared to other solution the embedding options are weak/non-existant
Q3: I'm not an SSO expert, sorry