Seat struts do both. They stiffen the relationship between the seat and the frame, but they increase weight transfer because you’re forming that much more direct connection from the seat to the bearing hangers.
I’ve seen karts get massive inside rear wheel lift with no struts on, but they can’t maintain that lift because the outside tire isn’t being driven into the track surface. The lift is coming purely from chassis flex in that case, not necessarily from weight transfer.
I would say 95% of sprint karts with Yamaha-level power or higher are going to be running at least one additional strut per side. Anything higher than Yamaha it’s basically a necessity. Sometimes you can run without them in Yamaha depending on track conditions. On a 206 I would test it both ways and see what it does. I know my dad runs without them, but given his experience (or lack thereof), I don’t know if he’s a good benchmark.
Regarding weight placement, Jim was basically spot-on. I’m smaller (5’7″, 130lbs) so we always start with our weights at the top of the seat.