Patches...

I haven't been modding for long so I don't know a couple things. When Nexon has their monthly patch updates, should I keep the modded rez files? Or will Nexon only add their new content on top of my old rez files.

I'm pretty sure it replaces your rez files, so you would have to replace them after the patch.
I think, wait for a second opinion lols.

Yeah, shocking's right. Keep the dtx file to make sure you can inject it whenever you want after or before patches.