I have a strong feeling that Rob is playing the long game with the Archer Clan.
He could be with us for years.
First and foremost, he needs to get his son safely born, hence his overprotectivenes for Helen.
He will then have legal care and control of two Archer heirs.
Next he will continue to undermine Helen's self confidence, until she becomes unstable and eventually, an unfit mother: possibly having an accident involving the children. Maybe even killing or injuring herself in the process.
If not dead, Helen is sectioned and / or seriously injured - and Rob will have total control of his family.
He seems to be very charming to Lady of the Manor, Elizabeth.,
who is certainty in dire need of a personable male partner.
Quick divorce from Helen. Marry Elizabeth - job done!