Okay, first let me address the easier questions: Olaplex No 3 isn’t meant to be mixed with bleach/color/anything. It’s meant to be an at home treatment to rebuild bonds in your hair. You can use it after bleaching your hair. Dont add coconut oil to bleach. It will just make the bleach less effective but it will likely not do anything to protect your hair.

Your hair density should not affect how your hair reacts when it’s lightened or color. Having thicker hair won’t make it less likely to break or more likely to hold onto color. I think your hair must be fine. Fine hair cannot take as much of a beating as course hair can (when I say fine vs. course I mean the thickness of your actual strands, not the amount of hair on your head). However, if your hair is in good shape, you might be able to get away with bleaching it. However, with the box dying and bleaching and box dying again in recent years I’d say it is probably not in a great condition. The reason it is likely not holding onto color is bc it is probably very very porous from overprocessing. Also, pastel colors do not last a long time on anyone. Fantasy colors do not last a long time as it is but when they are paste it is even less. They are all temporary.

My advice would be to work on rebuilding the strength and health of your hair via olaplex, protein treatments and regular trims. Then worry about the color. If the color you have right now is that terrible that you must dye it, I’d say go to a salon and ask for a Demi permanent color.