Feelings WILL develop when you open up your relationship. It is bound to happen. It could have been you.

You are both new to this and he didn't know how to navigate these feelings. Nobody knows in the beginning.

I'm concerned because she is about to go through a rough transitory period and may view my husband as a lifeline and develop an even greater emotional attachment

Yes. She needs him now. If that bothers you, I think you must reconsider your open relationship. Sexual bonding leads to emotional attachment, it is inevitable. It is part of the deal.

You don't have to like her, she is not for you to like. You may not trust her because you see her as someone who’s after your husband. Again, when you open up your marriage, you give up a lot of control and you have to trust your husband’s judgment and good intentions towards you and your marriage. If you don’t trust him, why did you open up?

If you need a high level of control over your marriage, you stay monogamous.

I get that you are uneasy with the way things are developing, but instead of doubling down on control, like you are doing, read up on poly. Both of you.

Start with the Smart Girl’s Guide to Polyamory. Start couple’s counseling in order to work on your communication and trust.