You may want to look into a plastic prep spray which is made to help paint adhere to plastics. Not sure if it would come out clear or cloudy as I have only used it before applying color on automotive plastics. I would also put it over your decal in case it wrinkles them if/when you test.
Have you considered a thin plastic sheet over the entire playfield glued at the edge of any hole to prevent dirt from getting in between the 2 layers?
BTW this project is great and I have enjoyed following the progress.

Be careful of direct sunlight. It can cause paint to dry too quickly. Overcast is good for painting and paint drying. High Humidity also will slow down dry time and can cause a whitish haze (trapped moisture).