I don't have anything to contribute, just more questions.
What about things like pre-tensioners [removes all slack from the seatbelt in the event of crash, right?] and load limiters that seatbelts [may] have? My understanding is that not all cars have those in rows other than the front one; but if there are, then how do these interact with harnessed seats, if at all [being from Europe I of course think ERL belts first]; and do these improve outcome with a booster?
My DD turns 5 in a month, and as soon as her baby brother needs her rear-facing seat [Britax MaxWay], which is bound to happen in a few months, I'll switch her over to a booster. She weights 19kg and therefore is over the common FF harness limit of 18kg here in Europe and I'm not getting the TWE.
My understanding is that here in Europe the plan for future is sort of that if it's FF harnessed seat, then it's installed with rigid isofix to eliminate misuse of at least the installation part. Currently isofix limits FF harness to 18kg, but with ISize there will probably be seats that allow at least few more kgs.
Did some googling while I typed this, and found
a study on boosters/integrated boosters and load limiters and pre-tensioners. Seems that at least on the test sleds load limiters/pretensioners do help with a booster.