Fallout – I accidentally replace vanilla Sequin Dress with a mod. Is there any way to get it back without reinstalling the game

The 'Argonians no longer gasp when leaving water' USKP fix is affected by the issues described in this USKP thread post in the Bethesda forums:

Skyrim Patch 1.6.89 introduced one critically bad bug:

Edited vanilla dialogue causes the audio and subtitles to fail to display. Papyrus fragments attached to failed dialogue will also fail to execute.

As a result, ALL edits to dialogue of any sort have been removed.

A temporary update for the USKP has been issued to address this, but it isn't entirely without use as several other bugs which can be safely fixed have been.

An easy game mechanic check to see if USKP is working is this (from v1.0 - 2012-04-07):

NPCs will no longer drop weapons or shields as separate objects, as these are never cleaned up which greatly adds to save bloating and litters the gameworld (the items will still appear to be dropped, but will access the corpse's inventory to be taken and be cleaned up with the corpse if left)

For other USKP changes you can check, see the Complete list of USKP fixes.

First things first before I get to the advice: Nexus Mod Manager is beta software. If you're experiencing problems with it, consider reporting the problem so that the coders and other users on the official beta feedback forum can read about it and any bugs can be fixed. If there are installation problems, they'll want to know about it and it's more likely to be fixed there than by asking here.

In the meantime, it might help to know that a mod manager is optional—you can install mods without ever touching Nexus Mod Manager. Since it's beta-quality software and you've probably just run into a bug, I'd advise learning how to cut out the middleware and install mods manually. Most mods are simple and it's just a matter of putting the right files in the right place.

Here's the simplified overview of how to install a mod manually:

1. Download the mod you want to install and unpack it to a temporary folder. You'll want to use 7zip for this, because it can handle the popular archiving formats that Skyrim modders pack their mods with. If you have the option of multiple file formats, pick "7zip", the one that end with .7z. Failing that, get the .zip file.

2. Look inside the freshly unzipped mod folder. Look for the readme file. This will tell you what you need to do to install the mod correctly.

   Sometimes the mod author will tell you to "put the files in your data folder" and you'll swear at them for not explaining what that is. They mean \Steam\SteamApps\common\skyrim\Data. If they tell you to put files in "your Data folder", they mean there.

   If they're not clear about what files to put there, you have to look a bit closer at the mod. If you have a folder named like the mod, don't put that folder in your Data folder—you want to put the contents in your Data folder. Same thing if your unpacked mod has a Data folder—you don't want to put that in your Data folder (which would end up with a Data/Data/[mod files] layout that doesn't work), you want to put what's inside it in your Data folder. You want to be looking for folders like textures and meshes, and files that end in .esp or .esm to dump into your Data folder.

   (If you're on Windows XP, you may want to disable UAC to simplify this process, since the game folder is in a UAC-protected area.)

3. Start Skyrim's launcher. If your mod had any .esp or .esm files (these are "plugin" files), you need to tell Skyrim to load these up when it starts. If there weren't any (for example, most texture mods that don't add new items won't have a plugin file), then Skyrim will find the mod files all by itself and you're done.

   So if there are plugin files, the launcher will let you activate them if you click on "Data Files". There will be a list of things, including Skyrim.esm, with checkboxes beside them. Make sure your new mod's plugin files have checkmarks, hit OK to close that window, and then click "Play" to enjoy. Once you've told Skyrim about a plugin file, it remembers, so you only have to do this once for a new mod, assuming it has plugin files.

If you want to read that all again using different words (which sometimes helps immensely in figuring this stuff out), there's a short article on the Nexus Wiki: How to install Skyrim mods. Manual installation is covered in item (8).

Finally, the Nexus forums and official Bethsoft Skyrim forums are very helpful when you run into trouble.