Much 'dramatic licence' with Rob's conversation with Henry, I think. Helen could easily have been in and out of the house in that time. Were I doing what Rob was doing, I'd have said 'so sorry Henry, I can only take Jack this time, please don't try to follow.. I do love you (as seat removal takes longer than planned).. byee'.
I can only guess this was not a newborn car seat (mostly last to near 1 as I recall), undone with one seatbelt click, but the next type, with much isofix and seatbelt confusion.
As for Rob loving Henry really... I've always sort of thought that too but, it wasn't the case when he took the cabin-bed ladder away and prevented Helen from going to him when crying, early on, that was awful. Plus the boarding school plan and many other instances when Henry was in the way of Rob's plans ad wishes. I do think he feels real affection for him, has some connection and could have been a good father (and all the rest) if whatever went horribly wrong to/with him hadn't happened.
That's the thing with personality disorders isn't it, they're mostly triggered as a result of trauma, neglect etc. Perhaps people have innate potential (no idea) but, the little I know is they are (also) made. So Rob is not 'evil' he has some shreds of a decent person, pushed aside by his warped personality.