Hi Seasnail, everyone.
My wife and I are new to polyamoury, but we are thinking of connecting with a very nice couple. I have an active imagination and have been picturing everything that can go wrong, (and everything that can go right too). I've been looking over posts on this site to give me a bit of preparation.
What I am saying is don't trust what I say as vast wisdom. I know less of this than you!
However, I am pretty good at nurturing and taking care of relationships.
It sounds to me that you are petty level headed. I'm not so sure about your husband (because of the affair) but apart from that he seems to be working at these things as well.
I was a very shy person for many years, and I decided I had to force myself to be more outgoing. Nowadays, I am pretty good at being friendly with people. But it is very, very hard to change habits of many years and it does not happen by accident. I think that you will have to work hard at that. The upside is that it is very rewarding to be able to connect more easily to people. (And even small improvements give meaningful emotional rewards.)
The code phrase idea is a clever one. You might also work on some non-verbal signals for when you would like to cuddle up as these would likely be less obtrusive.
I do think that you may wish to ask you husband not to add anything more to the mix until everyone settles into the current relationships a bit more.
You might wish to describe more about your relationship with your husband's girl friend. You've said she is in another city. Have your meetings with her so far been cool but correct, being friendly but awkward or what? I would say it is not needed for you to be best buddies with her, but it will be very awkward and difficult if the two of you can't maintain courtesy and respect. (Courtesy you can do if you try. Respect depends a lot on if they are worthy of respect of course.)
(By the way, if more experienced people think I'm out to lunch on any of this, please don't be afraid to jump in. I've got a thick skin.)
So my advice is:
-- work on your shyness. This will help you in countless ways in your life and will give you more confidence (likely) in this new world of relationships. However, it won't be quick and easy so it will be your long term plan.
-- you likely will need a lot of emotional support from your husband. Let him know this and be clear you will need lots of cuddling. Signals to allow you to tell him you need some 'tender loving care' subtly are good.
-- even so, you are likely going to be on a bit of an emotional rollercoaster for a while. It is not the end of the world if you feel terrible some times. Pamper yourself a bit and let it pass.
-- You might want to read the book "The Ethical Slut". It gives some good advice about relationships and the emotions of people new to polyamoury. Very level headed advice I thought. You must take responsibility to make your needs known. And when you set limits - and you find they are not working - it is OK to say "let's renegotiate something that works better".