May I be totally frank? It sounds, just from reading your post and not knowing you in any other way, like you are really resentful. I think, no matter how hard you try, that anger and bitterness will show through. I think you should not go because this is their joyful time, they do not need a damper on their shower. I mean, who cares if the shower is held in November or December? They were just probably trying to avoid the business of the holidays. I don't think you would even be making an issue of the date exzcept that you dislike the people and are hurt and sad for your own efforts at getting pregnant.
Why don't you graciously send a gift and your best wishes instead of attending? It might be easier for you and them, too. Your emotions can be raw if you are upset and discouraged over your efforts to conceive.