In your picture of the roof you can definitively see the roof is sagging from he weight of the AC unit allowing water to pool and possibly leak. You're going to need to unbolt and lift that AC unit off so you can replace the gasket under it. Once the unit is off you'll need to find a way to lift the roof from the inside and keep it up so water runs away from the AC. As you can see a 2x4 laid on it's side is not adequate to hold the weight, it's also really heavy for the application. Your idea of the steel tubing is good, but I would go with something like a 1 x 2 rectangle standing on edge with 3/16 walls. Every time I try to work with 1/4" it just seems too thick for most applications.
You might also try just pushing the roof up from the inside and slide a piece of 1x4 pine tapered at the ends between the aluminum roof and the 2x4 to put a bow in the roof.
Use a new gasket when you replace the AC unit and only tighten it down enough to compress the gasket about 1/2 inch. Do not over tighten and tighten it down until flat. If no leaks wait about 30-60 days then go tighten the four bolts another 1/4 to 1/2 turn each.