Is it possible to have a Peplink device running SpeedFusion on Site A to connect to FusionHub on AWS to access the internet, allowing the multiple WAN connections on the Peplink Device (Site A) to bond together, providing a faster Internet connection?
yes it is.
Is it possible to connect two FusionHubs, one on site A en one on site B, instead of using balance routers?
it should be possible (never tested by myself)
Yes that is possible.