and would you use a drop spreader or broadcast spread for a 50x100 lawn

Does it need to be watered in after application?
Yes
Should you apply before rain?
Unless you apply when it is raining, it will always be applied before it rains. Never apply any fertilizer or herbicide on wet grass.
Is heavy rain going to wash it away or does it have to be a light rain?
Yes a heavy rain can wash it down the storm drains. That is why you want to water it in with irrigation.

Always use a broadcast spreader. Drop spreaders are prone to striping unless you apply it at a half rate and go over the lawn twice - first time vertically, then horizontally. Remember, the more consistent the coverage, the better the preemergent will work.

How long can you wait to water it in? Is it okay to apply 12-24 hours before mother nature brings rain?

How much water is needed?

Is it okay to tranplant medium/large sized arbs into a treated area after a few days? If no, how many feet from the arb planting area should the application be stopped?

Answered questions by reading the bag of the bag after I bought the product.