This will need some testing:

• VS FF, PS shader: Read color1 value with specular enabled and disabled. Is it 0,0,0,0? Is it the color1 value of the vertex?
• VS shader, FF PS: Write color1 value with specular disabled, Write no color1 value with specular enabled
• VS FF: Use color0/color1 as source but vertex doesn't have it. We do 1,1,1,1 for color0 and 0,0,0,0 for color1. Docs say it should use the material color.
• Why do we only add the rgb values of the specular color to the final color and not alpha as well?

SPECULAR_ENABLE controls whether the specular value is added to the final color in the pixel shader. This also happens if color1 comes from a programmable vertex shader. If the PS uses a color that either the FF VS or a programmable VS doesnt output, it gets 0,0,0,1, regardless of whether its color0 or color1.

It also controls whether the FF vertex shader actually calculates a specular color as part of the lighting calculation or just passes the vertex specular color (color1) along (default is 0,0,0,1).

The D3DTA_CONSTANT default is 0.

Setting the material source to a vertex color that doesnt exist in the vertex declaration makes it use the material color instead (like the documentation says).

As an update for @Cambidd because those last two comments were notes for myself more than anything else:

There's PRs up that fix both the thrusters and the missing shadows, so the game will be returned to all its glory. I expect them to get merged next week or maybe the one after that.

Fixed