Can the shrimp be marinated overnight or even a few hours prior to grilling?

I would not do either. The marinade contains 1/4 cup of lime juice and marinating for more than a couple minutes would "cook" the shrimp and change the meat texture. You would have Ceviche now. Make up the marinade the day ahead if you wish, but don't add the shrimp until a couple minutes away from grilling. I wouldn't make the salsa either as the peaches and the mint could turn darker if it sits too long.