In the original film Gaslight the husband was trying to drive the wife mad, or get her and other people to think she was mad, to get her money. He convinced everyone she needed locking up.
I think knob is trying a similar tactic, not to get Helen committed for her money, but to gain ultimate power over her, her inheritance, farm etc. If she is considered barking then henwee will become his property under PR. Helen can't resist it as she will be having ongoing psychiatric care and no one will believe her. No one in any authority sorry Kirsty will think she is stable. She's not stable enough to plot her way out like julia Roberts in sleeping with the enemy. So she will have no choice but to surrender. At least for the forseeable future. And once the baby comes we'll will have full blown PND.