So we can deduce that man front right (no armband) is not wounded/convalescing?
No, I think that would taking things too far. There are all sorts of anomalies here so the absence of an armband is probably not that significant. What is interesting to me is that the group photo marks the transition between officers' badges of rank being worn at the cuff and then on the epaulettes. This happened towards the end the war but was not officially recognised until about 1919 or 1920. Since it involved the making of a completely new service dress jacket at the officer's own expense, this quite naturally started with the Brigade of Guards where most officers could afford the changes, and then spread to other Regiments and Corps. I think the chap seated on the right, without the arm band, is a Grenadiers officer, which supports the narrative above. He's not a gunner or sapper although they also wear grenades as collar badges, because of his much darker shirt which is characteristic of the Guards.
The other anomaly is that the captain on the left is wearing his Sam Browne in what would otherwise be an informal setting. Maybe he's just being old school, given that he is clearly older than the other men.
And yes I would agree that the car driver is the same man as the man seated on the left in the group photo. It is really annoying that his capbadge is so unclear, because I can't make out his regiment from the collar dogs alone. Collar dog badges frequently differ from the cap badge although they are often based on the same basic design. This doesn't apply to the Gunners or Sappers for instance.
