Inner panel how to deal with this rust?

I would use a sand blaster on the inside / back sides of anything that was not a wide soft outside surface panel. Blow / vacuum out the sand and then self etching epoxy all down in behind there. After that, Like others said, weld in replacement pieces if you can make / find them. If you cannot get to the rust on the back , get it blasted and sealed, it will slowly spread and it is a variable matter of time before it starts showing up.

I have heard there are flexible spray wands that allow you to spray epoxy inside structural areas with limited access. Have not found one yet.