"... use 1/2 ice and 1/2 water for a slower steam". I've read something like that many times and, unless it means the author wishes to delay the arrival of steam, it isn't scientifically valid.
Iced water will take longer to reach temperature X (above it's base temperature) than room temperature water. But once steam develops, the water is essentially at 100C ( 212 F) which is the temperature at which water turns to vapor at sea level. If you want to delay the arrival of steam, simply apply the heat more gradually or start the heat source later using whatever water temperature your kitchen faucet delivers.
I agree with the nuke approach to preparing your corn (amount of time depending on the power of your microwave - I nuke for 1 minute per ear, turning and giving each another minute or so) and would only add that as long as the water in your chafing dish is hot, the convected and radiated heat from the water and heated chafing dish surfaces will warm the corn and keep it that way with or without steam. Just be sure to start the heating process early enough to get the job done.
Microwaving unshucked ears adds flavor as the corn effectively steams in its husk. When cool enough to shuck, the silks come off easily. Another method is to cut the stem end off after nuking. Grab silk end firmly and pull out the cob, leaving the silks and husks behind. In your position, I would buy the corn a day before, and nuke it as soon as I got it home. You can arrange 3 ears in a triangle formation and nuke on high for 6-8 minutes. So it would take you about 2 hours. Leave them unshucked, keep them chilled till the day of the bbq, then use chafing trays with water and rack to heat them up before shucking. They taste just-cooked when reheated after storing unhusked after microwaving.
no I've not done that but I would.
but first I'd parboil 'em a tiny bit first then I'd use a rack if possible in the chafing dish and put the quartered cobs in on top of the rack sort of creating a steaming effect. there isn't usually a really hot sterno heat under a chafer and I'd use 1/2 ice and 1/2 water for a slower steam, then do a swap out when you see it getting low. do you have a really huge chafing dish?