Choosing texture2D over flixel_texture2D wasn't something I made a specific call on previously. While experimenting I used them both and didn't notice any change. It's a good question so I had a quick look around flixel source to see what the difference is. Couldn't find much but there is mention of it in this blog post, as follows https://haxeflixel.com/documentation/upgrade-guide-4-0-0/
using flixel_texture2D() in per-sprite shaders, the alpha and color transforms of a FlxSprite are already applied on the returned color
I'm only interested in applying the shader to the entire scene with this technique so may well leave it using the openfl texture2d. It might be more efficient even, though probably not to a perceptible level.
As for the swizzling, vec2(0.7) works too as you suggest. There's no particular reason I've not done that here but I do find you sacrifice readability doing it, without much gain. In this case it's just a leftover from rounds of experimentation, breaking and fixing the code. It's hard to debug this stuff and in general I've found being explicit is generally more useful as there's less to potentially go wrong.
I think there is swizzling in the code I got from the blog post at csantosbh.wordpress.com but in my opinion it does not help with understanding what is going on. I actually started rewriting it to be more explicit but that was more effort than worth since the copy pasta got me where I want to be for now.
I have since changed to use a smaller value for the alpha variable because I found that if you scale to extreme sizes 0.7 isn't enough.