I see you have built Peter's monostable circuit.
This is my effort. I've used three pots instead of fixed resistors for the RGB.
This shows what we are trying to add, eight "Broad Pulses" at half-line rate.
And here's what I got in practice after careful adjustment of the timing.
Even field.
And odd field.
Without this circuit in place there is no interlace apparent on my old Cossor 916, although my other set a Sony 9-90UB seems to manage without it, although I may be being fooled by its smaller screen size.