You have asked more than one question, so, according to enotes rules and regulations, I have edited it down to just focus on the characterisation in Trifles.
What is interesting about this play is that in a sense there is not just one protagonist - there are two - Mrs. Peters and Mrs. Hale. They are pitted against the male characters in the play: Hale, the County Attorney and The Sheriff. The antagonism between this play focussed on gender relations is built around the fact that the women are able to find the motive that the men are cluelessly looking for, in spite of the men underestimating their focus on "Trifles" - household clues that clearly reveal the reason for the crime.
To investigate this further, you will want to look at how the men mock the women and infer that they know nothing, only concerning themselves with "womanly" activities. A key example of this, and one that is referred to again and again at various points in the play to highlight the irony, concerns the quilt that Mrs. Peters and Mrs. Hale find. Mrs. Hale says of this quilt:
It's log cabin pattern. Pretty, isn't it? I wonder if she was goin' to quilt it or just knot it?
Note then that the men descend the stairs, and the Sherrif repeats her words, drawing a laugh from the men. It is highly crucial then, that straight away after this, whilst Mrs. Hale and Mrs. Peters are taking up their time with "little things" as Mrs. Hale says, that they find the motive in the piece of crooked sewing, that gives evidence of "anger, or - sudden feeling", as Mrs. Peters reports Mr. Henderson saying. Note how Mrs. Hale describes what she sees:
Mrs. Peters, look at this one. Here, this is the one she was working on, and look at the sewing! All the rest of it has been so nice and even. And look at this! It's all over the place! Why, it looks as if she didn't know what she was about!
The women, by engaging in their "trifles", have found the motive that the men have been looking for, whilst they have been stomping ineffectually all around the house. The answer was under their noses all the time, but needed a woman's knowledge to piece it together. It is this conflict that dictates that the female characters are the protagonists and the male characters the antagonists.
For Trifles: The protagonist is: Mrs Wright, the murderer.
The Antagonist: The male characters on the play, including the victim Mr.Wright.
I guess this is the modern (feminist) way of looking at the play.
hope this helps =)