Hens will lay when they lay, neither sooner nor later. "Start of Lay" for a particular breed is more like a strong suggestion, +/- a couple weeks, not a date to mark on the calendar in advance.
You can put down oyster shell any time.
There is no need to switch over to "layer" feed, and some decent reasons not to. Extra calcium won't cause birds to lay sooner.
Put your eyes on, wrap oyster shell in a heavy towel, between stiff leather, old denim, etc then crush. A car tire might work, with the weight of a car above it (depending on what you drive) - or a sledge, a tamping tool, etc. You aren't trying to powder it, you are trying to crush it. Odor is addressed (before crushing of course) by washing with a high pressure hose and then either baking in the oven, baking at the fire side, or baking in hot sun (last, least best option). A sieve for sorting is good - its just a stretched frame with some 1/4" HW cloth affixed, but if you haven't got one, aggitating crushed shell on a tilted board or similar will cause it to settle by weight - heaviest to the bottom, lightest (and smallest) to the top, just like panning for gold (or an archological dig)