for your 5 yr old, you could always get a ride safer vest, since you are nervous about booster use.
but most 5yr olds are fine for a booster as a spare seat (or even a main seat if they are total giants or a hwh seat isn't able to be afforded).
IMO, the vest would be easier to travel with than the booster, and you could sell it when you get back and get most of what you paid for it back, or just keep it if you will be traveling by plane again in the future.
with a kid who is able to be comfortable on the plane sleeping in a ff seat, I'd rethread the straps prior to the flight and put her ff if you want... turblance and even (god forbid) a plane crash is definitely MUCH different from a car accident and rf doesn't have the same benefits. For kids 1 and under, or who are riding in infant only seats, or whose parents don't want to have to adjust the straps, then it prevents misuse, then I would keep them rf.
If you know she won't complain, then you could avoid the potential fight with the attendant.
However, I do feel there is one benefit few think about (except us on this board, of course)
How likely would your 2yr old be to kick the seat of the person in front of her?
I'd rather her be kicking the back of her own seat than the back of the seat of the person in front of her.
Also, the seat will allow more shoulder room for the passenger (you or your hubby or older child) when the seat is rf.
So, if you do have it ff, you may find you want to place a child next to her instead of an adult, so that they aren't as squished. (like your oldest child, assuming they are patient, responsible, and get along with her).
But I agree putting her ff during the flight is something I'd consider doing to prevent arguing with attendants when traveling with so many in a group.
that's great that the rental place has Marathons, but, as you recognized yourself, you just don't know their history, can't guarantee that you wouldn't end up with an older model that is expired or has something broken on it, and it's just not worth the risk.
Oh... and with the rental car... if you can, find out what you will get ahead of time.
You may find you are more comfortable with the 5yr old in a hbb if it is a vehicle with locking shoulder retraters, because you can lock the shoulderbelt on the child (or have an older sibling do it if they are all sitting in the third row together).
My boys are both so in the habit and knowing that I like that it keeps the lapbelt snug on their thighs/hips that they lock their own seatbelts in their dad's car that has them. (of course, they don't know my other reason is because it limits the amount of fighting they are able to do)