There are books written on how to help a partner become comfortable and enjoy sailing. I am not of "female persuasion", so you may get better advice from the ladies on this site.
But in my 15 years of experience owning 3 different boats, I have learned to ALWAYS respect and listen to my wifes wishes on board (except where an immediate decision on safety is needed, when as the captain I reserve the right to manage the boat). The one time I suggested we stay out a little longer after she thought we should return to the dock (last sailing day before leaving the boat for a month, and it's so hard to leave the boat....), my wife lost her balance when a large wave hit, fell and bruised her arm quite badly--I learned my lesson!
I try to choose good sailing days (so you might have waited for a warmer/sunnier day with lighter winds til she is more comfortable with the boat). I have also learned to work with my wife as sailing "partner" rather than as crew/go-fer. I also try to ensure we share tasks ie. doing what I can to make her day on the water as pleasant as possible as well, and minimizing the "work" aspect of preparations and sailing.
My wife now likes taking the helm sometimes, but other times she doesn't want to--I don't understand yet what determines that (I always love being on the helm!), so I offer, and respond accordingly. I also bite my tongue alot regarding "sail trim" and have to remind myself that I have more sailing experience/"feel for the boat"; as long as everything is generally all right, I overlook a sagging telltale or trim problem.
We also try to alternate with time on the boat and doing other things--even though it almost kills me to go for a walk or other activity when the sun/wind are "just right" for a great evening sail.
Finally, about your idea of adding more sail or similar steps to show what the boat can do...I think this idea has merit, but ONLY after explaining carefully that you are doing this for a short while, and describing all the controls you have available to make the necessary adjustments to manage it if it gets overpowered--e.g. lowering the traveller, adding backstay, boomvang, releasing sheets, rounding up, etc., so she and others are sure they won't capsize or worse. And even then, I would only do it on a nice day, when all has gone well, everyone is in a good mood, and again in somewhat moderate conditions when you are sure of what you and the boat are doing--and then only with everyone's permission. Then showing how lowering the traveller reduces heel, or furling the headsail restores equilibrium can be effective in showing that there are ways to manage when the wind or waves increase.
I'm sure others also have better ideas, and most of us would do the things I've described anyways. However, I would be interested in what others have to say about this important topic.