Great! Thank you for the reply - I think the seat belt routing guidance was what we were missing.
I flew with both of my sons from about 3 months old in the R44. When they were babies, we put the Max-Cosi car seat in the rear (initially rearwards when they were tiny). The seat belts in an R44 won’t extend far enough to wrap around behind the seat in the strap guides. I used two solutions 1) bought seat belt extensions or 2) ran the seat belts around the hard-points on each side and under the seat. I found the second method the easiest and most secure. (photo below)
When they were older, we used this seat (can't recall the brand/model), but it disassembled into two pieces and was an exact fit for the rear under-seat storage compartments, if by chance you only had the kids on one leg of a flight and wanted it out of the way.