I don't exactly understand your shot, but avoid having the green screen reflected in water at all costs. Even if it means you have to roto the shot instead of key it, that'll be way way easier than trying to remove a bunch of green reflections. You can try tons of different tricks to recolor...
I learned lighting in theatre. On stage we do all sorts of crazy non-realistic lighting so constant experimentation is going on. Nearly every show calls for a totally new way of achieving a look. That background gave me a very solid understanding of light. I actually fell into film lighting...
Try this.
Create an Adjustment Layer above the element you want to spaghettify. Apply the effect Distort\Transform to the Adjustment Layer. Create a mask around the black hole. The edge of this mask will be where the stretching begins. Uncheck Uniform Scale in the Transform effect and then...
On the paint topic, you will need to do more than one coat with green, even if it's Rosco. When estimating the paint you'll need most places say that one gallon will get you 350 sq feet. Personally I have found that 300 sq feet is best to estimate when you need a good even coat with perfect...
Don't use muslin underwater. It's going to soak up the water and relax in weird ways, change it's color to a much darker green, and generally give you a headache. Use Spandex instead. Rosebrand carries keying spandex. That'll make your life much easier, it will stay taught on the frame and...