How to insert tf frames into HectorSLAM tf structure - using data from the Pixhawk 4 mini's IMU

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I can answer Q1, Q2 is solely dependent on your application and I'm not going to go into application specifics.

How do I insert frames

I'm glad you asked! There's a great tutorial here (http://wiki.ros.org/tf2/Tutorials/Add...) that discusses it. The core bit is that in your node (or create a new node) you create a Transform message with the parent (root) and child (child) of the transformation tree frames with their relative current position and orientation. An example from your tree is that the base_link is the parent to laser, so something is publish that frame via code similar to this tutorial to TF. So if you want to add a base_stabilized from base_link, you just need to make a message via that tutorial publishing it with whatever pose information is appropriate.