If you were to use seat belt only FF, how much benefit would the child receive from the REACT system in case of a crash?
ETA: And how can I tell if there is any interference between the LATCH anchors and the seat belt? I am trying to figure out if I can do this in my 09 Subaru Forester, am not a tech and have no clue, and Subaru referred me here!
REACT was designed specifically for rigid LATCH installation. (The rigid LATCH is actually part of the REACT safety system.)
When there is interference between LATCH and seatbelt essentially you can't properly use one due to the other.
For instance, I will use "o" to indicate the lower anchor, "-" to indicate the point connecting the lower anchors in a dedicated seating position, "." to show empty space and "x" to show the anchor point of the seat belt and the buckle. (Imagine first "x" is your seatbelt anchor point and your 2nd "x" is your buckle.)
x..o----o...x = no interference
x..o---x-o = interference
In the 2nd example, you couldn't readily buckle the belt because of the lower anchors.
Interference isn't always that cut and dried, but the basic idea is that if one system interferes with the use of the other, there is interference. In most vehicles, standardized seating positions have seatbelts that are positioned without interference. The seating positions where you get interference are typically when you have an offset center LATCH position that overlaps slightly with the seatbelts.
(Side note that sometimes outboard LATCH and seatbelt do have interference with the center seatbelt, so this may be a consideration in some vehicles, but in general, a dedicated LATCH position doesn't typically have problem with seatbelt interference.)
But this couldn't happen right? In a car new enough to have LATCH, you have a seat belt that locks somehow. Either you have a switchable retractor or you have a locking latch plate.
There is one model vehicle in Canada that does have ELR only seatbelts and LATCH. The model is one that is only available in Europe and Canada, and as neither country has lockability standards, the vehicle manufacturer has chosen to leave the seatbelts as an ELR only retractor. (I was extremely shocked when I discovered this, and I've only seen it once so far, but it may be possible to run in to this if somebody imports a vehicle or in Canada it could potentially be slightly more common. The vehicle manual in this case also essentially said to always use LATCH when installing a car seat.)