Hi there
if you GET an opportunity with the v2 API and specify that you want the party embedded, the API returns the organisation (if there is one) as that party. It does not return details for the primary person associated with the lead.
A common reason for wanting to grab the party this is that you want to get the email address for the specific customer for this lead. Unfortunately you don't get that unless you've put the email address in the organisation. You presumably have to request the additional parties in order to get the email address(es).
So...
1. I suggest you document it. I don't see this explained in the documentation, but perhaps you could clarify there that only a single party will be embedded and that it will be the organisation if there is one.
2. When retrieving an opportunity, would be nice to be able to embed all people associated with the lead, not just a single organisation.
Best wishes
Alan