In the past, on similar offers:
Even though they say that the credit may take a while to arrive, it has arrived (for me, at least) almost instantly, so that I've been surprised when all of the sudden it is being used to discount whatever book I'm looking at at the moment. Also, I've gotten an email confirming the credit pretty quickly as well.
As for the pre-orders - the T&Cs (see the OP) say:
- Offer applies to all Kindle eBooks made available through Amazon.com and smile.amazon.com, including pre-orders that are fulfilled within the offer period.
Which seems pretty clear, but you never know with Amazon...
I have had uniformly good experience with them counting books purchased during the offer period, but before I activated, which is another provision I was doubtful of the first time this type of offer showed up. But I've never had a pre-order come "live" during a promotion period so can't speak to that one.