I can't answer all of your questions, but I will respond about getting your placeholder the day before the last full day.
My advice is to get it on the last full day, not the day before the last full day. I got our placeholder relatively early in our cruise, and later when I wanted to book a cruise that would have fallen exactly 2 years past one of the days within that cruise, I wasn't allowed to use the credit on that one, because I had booked the placeholder mid-cruise rather than on the last day: they pointed out to me which day I had actually purchased the placeholder, & wouldn't budge on letting me use it, even though I had purchased it mid-cruise. So every day counts, & booking earlier just gives you fewer cruises that you can apply the credit to. That burns if you tend to travel at a certain time of year, which makes this scenario more likely.