Are you able to please give any guidance on which manufacturers allow both seat belt and LATCH? I got nowhere when I called Subaru a while back. They directed me here. The OP and I both have Mazdas.
If it's not prohibited in your vehicle manual, and there is no interference between the seatbelt and lower anchors, then we allow it. The lower anchor system and seatbelt system are independent of each other, so in the case of a seat with rigid LATCH such as the foonf, there aren't any unknown variables.
Previously the instruction in the car-seat world has always been to never use both - partly because of not knowing how one would influence the other, and partly because car seat manufacturers weren't testing that way.
Clek does test the foonf using both lower anchors and seatbelt, which is why we allow the use of both.
When boosters with LATCH first came out there were similar questions so I think there will be a higher incidence of questions regarding this until it is more widely done.