Make your marsala with thigh, rather than breast. Thighs stand up to a lot more abuse. Breast dries out and gets rubbery.
On the reheating thing, the rule is to heat gently, and to as low a temperature as possible. Food is perceived as hot at around 120F.
That brings up another rule which is the "holding" rule. If you're going to hold the chicken warm for any length of time (more than a couple of hours, say), you should hold at or above 140F.
Caterers use special holding ovens, generically called "Cres Cors" and "Cambros" which hold at exactly the right temperature. Just sayin' is all. Obviously, if you had that stuff available to you, you wouldn't be asking.
Here are some practical suggestions:
If possible, Heat a chafing dish worth of chicken for the buffet (or however you're planning your buffet service); another for backup/refill; and continue heating as necessary.
If you can't spare the time (or the help) to have someone continually reheating n the kitchen, you can also hold food hot (it's worth repeating, reheat as gently as possible) in a warm oven, around 200F. Of course you'll want to keep the food well covered.
No oven available? Hold the pans of hot chicken in foam or insulated fiberglass coolers. As long as the top is tightly closed, an ordinary drinks cooler will hold food hot for several hours. Lay some towels on the bottom of the chest to keep from scorching it. And to whatever extent possible, pack the empty space with crumpled newspaper.
Nothing refreshes the appearance of held food like fresh herbs. Garnish your platters with plenty of freshly chopped parsley before placing on the buffet.