"They are women and they are happy."
Translation:הן נשים והן שמחות.
No, the ending of שמחות shows that it's a feminine word. It just happens to share the ים ending with many masculine words.
why can't we say הֵנָה instead of הֵן? Also, why can't they be after the word like this: נשים הן
הנה simply doesn't mean the same thing as הן, And the two can't be replaced. Secondly, by replacing the word order you'd be referring to women in general, instead of the specific group the sentence refers to
Because הן means are and הנה doesn't. As for the second question, it's because it changes the meaning. הן נשים means they are women, נשים הן means women are.