In reply: AN's NW weathers aren't the same as the originals. The edits are almost universally:
1. HDR tweaks. This is to prevent the nuclear white-out that plagues weathers without them.
2. Colour tinting changes. This is to bring the tinting of NW's weathers into line with the rest of the weathers in vanilla and the other weather mods - NW's original weathers had a very noticeable yellow tint to them.
3. Occasional aethstetic tweaks - a few NW weathers are pretty ugly compared to others, and have had colours and textures changed to make them look better.
So to answer your second question, the vast majority of NW (and other) weathers have their original textures. A few have had textures replaced with very high quality textures Lprman made for me.
You can get NW's yellow tint back by changing the ini values for Colour Tints, try raising the red and lowering the blur multipliers.
As for the weather script, AN's weather script is completely unique. I believe this is stated in the readme. To put it bluntly, NW's script is a pile of rubbish, and there's no competition. I realise I'm biased, but there's no excuse for releasing something that broken and never fixing it through 2.1.3 versions of the mod. Sure, AN does have bugs, but it's trying to do things that are a heck of a lot more complicated, and I work hard to fix them when reported.
Well, at last (neally two month ago), I decided to just use AN for RL, NI and the weather changing system, which (especially the first two) are only full featured possible in AN now. I havn't played TES4 since 20th Jan because I didn't take the hard disk installed TES4 with my laptop. just recently I began browsing TES4 mods again and remember to post here.
I'm not very farmiliar with programming scripts, and they are not very similar to numerical progam which I know a little more.
I know AN's script must be unique at least because it combines several types of weather. The script have a sorting part in the beginning to archieve weather type selecting, and then dealing with classified combining weather with a unify system. I'm sorry that I havn't understood the major script for weather changing and don't want to yet now.
But I thought a system combining there types of weather may cause each type (at least two) can't show its best performance, both in changing and scene. For example, using ASW type when sunset the screen is too red to look; using NW type when sunny sky is pure blue, and there is a cloudlees sky belonging to cloudy weather type which make street lamps light in day; using EW it didn't make me some uncomforable and was similar to vanilla, so I don't remember much now, but when storm in day bright overcast sky and white hill shadow is also a pity.
Each type of weather may have its own "equilibrium point", so a unify system including changing, light and something else may not suit for every types. Perhaps it is better to give different type a little different treating, and then I realized for AN the tweak method is only modifying ini file by user himself.
Adjusting weather effect by setting ini is really a boring and weariness work, especially terrible for eyes. At last I found at first I am pleased with vanilla weather effect. The only reason I looking for a weather mod is the vanilla changing system, which may hold weather and change until I get in and the out of a house or cave. So at least disabling the three new weather types is OK for me. Now just simple effect is enough.
At least, just for RL, NI and reasonable changing, it worth using AN. And I like this mod for its wonderful RL & NI effect, though there are nealy two hundred Mbytes BSA data unused for me.