Mile offers fluctuate, with no discernable pattern. What you will generally see is that if the miles offered goes up or down, it tracks with the price going up or down.
The majority of my bookings are non-refundable once purchased. Changing or cancelling is not an option. This will be clearly spelled out during the booking process.
You may also find refundable options, generally at a higher cost than a non-refundable.
I would always recommend getting to the final screen (before entering CC information) to see the total price (which may include additional resort fees, etc), then compare the cost to what it would cost to book direct.