Yes you can float on top of red guard before tiling.
If you want a perfectly flat wall and assuming you are tiling then the proper way to do this is to put some 1/2" thick strips vertically make sure the strips are plumb and put a level across the two strips and make sure they are level with each other. Then you can put a fair amount of thinset and screed between the two strips once the thinset sets up a little you can remove the two strips and fill in.
Depending on your tolerance for perfection and how much extra time you want to spend getting your tile flat, you could also just use modified thinset and your level as a screed to make the areas you've indicated flat across the width of the wall.
I'd probably still do the red guard again on top of the float as thinset is porous and will absorb water but the water would eventually come out and you have a waterproof barrier so you don't have to do that.
I'd go with a polymer modified thinset for better bonding when flattening it out out.
I would use epoxy grout as well for a better waterproof system and to avoid grout that stains or needs to be sealed all the time.
This is the screed set I use.