Garage Floor - preparing for paint, stubborn stains?

I am preparing a garage floor for epoxy paint.

There are old grease stains that I cannot remove. I have used a garage floor degreaser, with scrub brushes and a pressure washer. DIYnetwork suggested dishwasher detergent, laundry pre-treater, etc. but I am concerned that putting too many competing chemicals might cause toxic fumes.

After pressure washing, there is a large light brown stain remaining (the oil/ grease had been so thick we actually scraped it away before degreasing).

Is there a way to test the surface to see if the stain still contains oils that would keep the epoxy from sticking?

The garage kit includes an etching chemical, and a two-part paint (pigment and activator).

There are also some shiny black paint stains (drips) that even the pressure washer couldn't blast off. I actually pitted the concrete attempting it, so more pressure isn't an option.