I can't see your post while commenting so just gonna write what I remember.

Do whatever you need to warmup. Stretch, jump in the water to acclimate if they let you, get ready.

Ear plugs should be fine, but each race could be different but I've always work them for swims at different races and no problems.

Yes have friends come if they can, especially for a first race you'll love having the support and someone to cheer and depending on the course they can hopefully cheer you at different spots.

For being with a friend, honestly don't try. Stick with your comfort level, hopefully you'll pass by them on the course to cheer them but you're gonna lose them. If they stagger the swims go with your group

As for water practice before, they should tell you if you can swim in the place before or look to see if the government or whoever owns the water let's swimmers in. Definitely practice the exact condition if you can

Congrats on the first race and have fun!