If your wife isn't on board, she isn't on board. Do not try to coerce her, persuade her, trick her, etc.
"She holds that she would be miserable for the rest of her life if there was another woman in the house that I was married to."
That is entirely valid. Plural marriage is not for everyone. Respect that, or you might find yourself with 0 wives.
" She doesn't feel it is the direction she wants, therefore it can't be for us."
Again, it isn't for "us" if she isn't on board; it might be for *you*, but if she isn't interested, it isn't "for us". Recognize that, or you might find yourself with 0 wives.
"As you can tell, she is rather strong willed (which is not necessarily a bad thing, except that now I think it is starting to make her disrespect my headship over her)"
Whoa, whoa, whoa. Your wife is presumably an adult; if she does not want to enter into a plural marriage, that is fully her right. She is not disrespecting you by not wanting to jump into something that you've decided after the fact. Especially something as radical as bringing home another wife. Polygamy is, and always has been, in the minority. Expecting a random woman to just be fine with it, and thinking that she is disrespecting you for not going "oh yes baby, let's do it, let's do it right now, I'm fully on board" is more you disrespecting her agency than anything.