In Slade, how do I transfer/copy and paste textures.
The question is where you want to copy the textures to. By using the TEXTURE1 lump, you can add all your textures into that lump. I guess it makes no sense to create two texture lumps with the same name. By the way the ID group needed the TEXTURE2 lump because there is a shareware and a full version of Doom 1. There is no TEXTURE2 lump in Doom 2, Plutonia, TNT, including Hexen as well, because POOT is the same as FOREST05. But if you want to copy from TEXTURE1 to TEXTURE2, you can do that by right clicking on several previously selected textures and simply copy them then paste them to the other texture list.
Also, is there any way to replace all textures and flats in a WAD across all maps simultaneously?
I don't think so. But you can use the search & replace function of the doom builder 2 or the replace function of a hex editor. You can edit the wad file but bear in mind that the name of textures should be 8 characters long. If the name is shorter than 8 characters, fill the remaining space with 00. That's all.