I don't think you will default to the higher rate. No one rents for exactly 7 days, down to the minute or even the hour. Yet reservations are booked that way. I am frankly surprised by what you're describing here because a 6 day, 22 hour rental should be charged the 7 day rate because a few hours after you cleared 6 days exactly their hourly rate exceeded the price for a full day and it defaults to the 7 day rate. That is very odd that 6d22h is coming out higher than 7d00h. Late flights are common and returning a few hours before departure time is routine.
I'd recommend you book the full 7 days if that's what gets you the best rate and I really doubt returning the car 2 hours prior to the drive-out time will result in the rate resetting. I've only heard of this occurring for people who booked a rental that required renting part of a weekend day and then returning early and not including that weekend day. In that case it defaults back to a non-weekend rate. I've also heard it happen to people who had a 6-7 day rental and got the benefit of a weekly rate but then return after just 5 days. It then defaults to 5 single days rates which is more than the weekly rate. But I've definitely not experienced what you've described.
BobK/Orlando