Hello Vincent,
I've recently completed a posthumous portrait for a major client in Singapore, and i must applaud your courage to undertake a challenging portrait such as this.
Like you, i was given very few photo resources - and one video cd of the subject. All the photos were only good enough to get a visual of how this person looked, but none of them had the right feel and pose for the painting work.
Through the video CD I managed to capture one image frame which was just what I was looking for. The trouble was it was only a shoulder length image!
So with the image, and the other reference photos given i had to communicate constantly with my client, and even did an actual size charcoal draft to present the complete idea to them . That got their approval and the rest is history.
My point to drive home with you is that with all these photographs that you have, it's important to talk to them and have them decide which one best represents their
collective memory of the subject.
Even when you don't have the complete image you need, build one with a charcoal / oil study with their consultation. Once they're happy with that study, you naturally have their approval to go ahead with the final image. Hope this is helpful information to you.