Pretty much the same going with Spider-mobile. Different people riding about in it and of course is transparency, namely on the windshield. I'm using the #e0e0e0 index on the alpha channel for translucent with this one, the standard d7 wasn't cutting it. I had kept some of the original settings of that model but scrapped them and I'll tell you why then.
Doing what you're doing with the slider breaks the model. The game doesn't handle alpha channels that well. The first game doesn't handle them at all in game correctly. The second is fussy. Using the slider method looks nice but the issues this creates are 1.) if you are using alternate skins that call for that piece to be alpha'd out completely you can't do it anymore, it no longer uses the alpha channel on the texture 2.) you can longer use Stencils since they use the texture as well and those are essential functions of this game engine, for you they're going to save you the issue where if a model isn't textured to both sides it will simply vanish if looked at from the wrong angle and 3.) in game break various texture properties, usually this is centered around the shading and reflective properties as the game engine utilizes the alpha channel on the texture for those.
If you look at any of the nifs that come with them with this game none of them use that slider property. Any use the texture property instead (Eye Of The Reich's glass dome is a standard example) coupled with a Stencil.
The standard is as follows, this is what I ended up scrapping all parts of Spider-mobile and reverting to after having tried a few things:
On NiMaterialProperty Alpha is 1.0000 and Glossiness 10.0000.
This is important because of the stuff I mentioned and conforming this to the how the game renders the other characters, stage elements, FX and otherwise your's will interact with. You "can" use the slider thing but I don't suggest it if going the alternate texture route, only if the nif is staying as is though the stanard 1.0000 method works just as well so there's no need for it.
So with those settings color indexes for Third Reich are:
Ambient: #ffffff (1.000)
Diffuse: #ffffff (1.000)
Specular: #e5e5e5 (0.900)
Emissive: #3b3b3b (0.235)
These are what the rest of the game uses. The various tools that convert nifs and keyframes from the first game to the second have a checkbox to lighten the values and what that does is set those settings to those values.
To NiAlphaProperty:
There's a few things that can be done with this but game standard is as follows which again causes the engine to rely on what is on the alpha channel of the texture itself, that being the differance there between black (#000000) as not visible at all to white (#ffffff) as fully visible and the range between deciding level of transparency. That one's a small margin but I gave you the two index range used here already, so what you would enter into the palette of a paint program before filling in other words.
Enable Blending: Y
Source Blend Mode: Src Alpha
Destination Blend Mode: Inv Src Alpha
Enable Testing: Y
Alpha Test Function: Greater (than >)
Alpha Test Threshold: 200
Sort Triangles: No
There's a technical run down if you're interested but these are just the standard settings this game works with. And again for the reason of modding options noting the function you want to achieve can be broken by not doing it this exact way, and trust me I played with this one too many times. You'll otherwise see when I release Spider-mobile and can look at the Nif. I had a slider determined windshield and it seemed fine and same thing you're doing, different riders, passengers as well parts of the vehicle could be stripped out by skinners were my objective ended up wrecked in the game itself. My drivers would show up as ghosts at time, the window could not vanish if I wanted it too, the reflections were broken in various areas depending on how in game light hit them usually resulting in them turning into white blobs, so on. Because the game uses the above indexes so when it read the unusual values I had entered in it didn't know how to have elements react with one another when one runs against the grain so to speak.
Anyways hope all that is of use, I have a lot of experience with that transparency as I said since I do multi-texture nifs constantly. One other thing, Blender puts these in there and I see them in the examples you have me look at, if there is a NiSpecularProperty in a piece get rid of it. This game engine doesn't deal with that function, relies on texture as well and it will break you're alpha and leave you searching for what happened. Stencil will either fix or break depending on the 3d model itself. I know how to set those up in the 3d modelling and texture assignment process outside the game and Nifskope but if you're importing this is something you'll have to just test and see what works. The short version of it is this is dependent on whether the 3d model had one or two sided texturing baked in to begin with. When importing works from around the web mind those baked into model settings, much of them get imported with the model and can end up being the mysterious cause of your woes.
