Indeed. If I'm right in saying, Jenova inside the Shinra Headquarters is the main body of Jenova, and Sephiroth summons the remaining parts of Jenova to reunite with the head that he posesses at the Northern Crater, so he can "carry out" his Spirit Energy and vessel plans with her once Meteor hits.

The Sephiroth you follow upto the Northern Crater is the main body from the Shinra HQ as stated. Presumably the fact it didn't act until Cloud and the party got there isn't just coincidence- when Cloud looked through the window (and this is all guess work but it seems logical) it may have read Cloud's memory and mimiced Sephiroth from it... possibly.

The different parts of Jenova you fight are as far as I know, all the main body aside from Synthesis which is the result of the Reunion (but not full strength, as some of the Sephiroth clones didn't make it and Cloud obviously never surrendered himself to Jenova.)