Scotchguarding dress

I didn't, but our ceremony was outside on a slate patio, I walked around paved areas and grass for photos, and my train honestly wasn't that dirty at the end of the night. It certainly wasn't sparkling white, but it wasn't black either. Just carry, or have someone carry, your train for you whenever you're walking around for photos, and only let it drag behind you when you're actually walking down and up the aisle for the ceremony.

I would worry that it would make the tulle stiff

Most of my photos were outdoors on grass and paved areas and I just had someone help me carry the train and veil or I just picked them and draped them on my arm. I didn't have any problem with keeping it clean this way.

I never even thought about this. We will be outside for the ceremony and photos only so I'm not super worried about it honestly. I'll probably get someone to help me pick it up when walking around (aside from going up and down the aisle, and I won't be worried about it at all once its bustled. I don't think we will be outside long enough for their to be any damage to the dress a dry cleaner couldn't fix.

I honestly wouldn't take the chance on doing anything beforehand - you can always get it dry cleaned after but you can;t fix it if the scotchguard ruins it in advance

I didn't, and mine was super dirty by the end of the night. However, I got married in a garden where the walkway was dirt and took many photos in the garden standing in the grass. Honestly, no one noticed and they were able to get it cleaned while preserving it.