For your first question, you could do that: create a tga with alpha 50%, then apply it with a “decal” blend effect as described in Texture Modes. But I don’t know if that’s really what you want. Probably, you will get better results just from using the default Modulate mode, which requires no particular alpha.
To slide the textures independently of each other, set a different setTexOffset value for each one, as described in Texture Transforms. You could do this over time with a task, or you could create a LerpTexOffsetInterval to do it more-or-less automatically. The LerpTexOffsetInterval isn’t specifically documented, but it’s basically the same as all of the other lerp intervals described in Lerp Intervals.
David