Looking at the way prices vary day by day on Iceland Air and Norwegian Air (they vary depending on July 6,7,8,9 etc.) I am thinking the prices go up as the planes fill up. Thus the tickets will likely be more expensive if you wait until June. If one day's plane isn't filling up they could possibly reduce the fare, but that may not be on the day you want. You are tied to a specific date to fly--either July 6 ( assuming she will go to work the next day) or a date 2-4 days later, depending how much extra time she gets.
If you wait to book and the flights cost more, that isn't your only expense. If she does get 3-4 extra days, you'll have more expenses in travel, meals and hotels for those days, so the total extra cost could $1000 or so. Are you comfortable with that?
Right now prices for July 6 are $633 on Iceland Air and $705 on Norwegian (and $639 on July 7 with them, but that date won't work.)
If it was me I would book that now for peace of mind, and build credit with my job/boss/co-workers by not taking the extra time off. June 17 to July 6 is almost three weeks, a very nice trip (we always go for only two).
