@OnceUponaTime
I can sense how deeply traumatic this event is going to be for you, but it shouldn't have to be. I realise we all have different symptoms and experience T in different ways, but an event like the one you'll be attending shouldn't really be envoking fear at this level. With protection, there really is nothing to worry about (arguably without protection - depending on what else happens whilst there).
If it was me, I'd have earplugs on hand to use in case something happened and I needed them. I wouldn't go with them already inserted though. But that's just me and that's not what I'd advise you to do. This is mainly because you have a deep seated fear of sound that no one on here can help you with in a week. With protection you'll be absolutely fine; just try not to hyperanalyse everything whilst you're there, because this is the rut many people find themselves in. Especially when they go out for the first time with an extreme anxiety to sound.
Also, with double protection you are going to spend a lot of time listening to your T at its loudest intensity. There will be little sound to camouflage it. Don't perceive this as an increase because it's not. After removing protection (that's high for the intended use) you may find this heightened intensity stays around for a while. This is not because your T has worsened, it is entirely perception based, and will return to normal after a while. Sometimes the next day after sleep. This happened to me, many times, back when I was phonophobic and overprotecting myself at every opportunity. It's usually best to match the protection level to the noise you're exposing yourself to, but again, I'll reiterate that in your case you may be better off using any protection you feel necessary because you're not mentally ready. And being mentally ready is a monumental step.
The other thing to factor in is that it's all outdoors. The sound won't amplify as it reverberates off any walls. It will decay instead. And also, the occlusion effect is nothing to worry about, at all, unless you plan on singing and shouting all evening.
The main thing is not to worry. Put your ear plugs in and have a great time. The risk of making your condition worse is so minuscule it's not even worth worrying about.
Finally, congratulations to you and your husband.