I really don't velcro will hold in the wind created by a vehicle on the road. You could be the test case for this application.
Yes VHB is a great adhesive but will be impossible to remove later on. The double hook velcro is very strong. Problem with Velcro exposed long term to sunlight is the nylon used to make it. I know of no UV resistant Velcro. The VHB itself will not damage fiberglass but trying to remove it would.
The magnets will not interfere with the solar panel at all. The only thing it will interfere with would be a compass either electronic with a magnetic sensor or mechanical within about a foot of the magnet. Magnets do not interfere with electronics.
Something like this would be perfect and could be removed without having to use a crane. 39 pounds of force 1" diameter with countersunk hole in center. Use 304 stainless countersunk screw with with the panel sandwiched between two 304 stainless flat washers and stainless nylock nut. Reason to use 304 stainless hardware is that it is non magnetic. Looks like a 10-32 or 10-24 screw can fit hole in magnet.
My idea was based on the flexible panel having some type of mounting holes. It is the reverse of what you described with the magnets on the outside. Your way should work if your magnets are strong enough and the distance between magnet and plate is not too far.
That 39 pound is likely meaning axial force, pulling straight off. Be aware, force to slide the magnet along and then free of its attached to surface will be considerable less.