Hey Mootish,
The gaping can be normal, assuming you have proper temp gradients, which you said that you did. Just like you mentioned with your beardeds it is a way that they get rid of excess body heat. If you do not have enough temperature gradient it can be a sign that there is not a place in the tank for them to cool off. If you do then it is just a sign they are being lazy. I have my small Egyptian in a 4x2 ft enclosure, and sometimes he STILL sits and pants (gapes) under a basking spot.
I think for a short term, the 55 will probably be better. It will at least allow a much larger temp gradient than the 29 gallon. The main thing to double check though, is that the heat is getting to the floor, since 55 gallon tanks are very tall. Also, this will effect the amount of UVB that will reach the lizard. My lizard basks on a high spot he has, so he takes care of that himself. I am not sure I would recommend putting a high climbing spot in there for your guy. He really does look quite dehydrated. If he tolerates handling without over stressing you could hold him closer to the light for 10 minutes a day, or carefully supplement him in the short term. Once you build his real enclosure then you can make sure the lighting is more properly spaced! 
The scratching/digging at the walls is sometimes just something they DO. However it can also be a sign of stress. Since "his" housing is glass on all walls, you might want to try covering over at least 3 sides of the tank, so he doesn't have to worry about them as much. Some people cover all sides for the first bit until the Uro acclimates, but if he was in a pet shop he was probably used to people walking by. I have found my Uro does much better with only one open side to his enclosure. He still "glass dances" (you will know what this is when you see it), and claws occasionally. Often this is his way of telling me he doesn't want whatever food is in his cage, he wants something else. Sometimes it is just something he seems to do for amusement.
Feeding him high quality greens, since he is eating them, is great. It won't take the place of good vet care, which you are in the process of getting, but it is a good sign. Leaving the water on the leaves of the greens after washing them is also a good idea, might get some additional hydration into him. Though that is my "can't hurt" opinion. That amount of extra humidity won't cause any harm. A "humid" hide also might help, particularly with his skin and shedding.
I don't remember if you mentioned about his age, but at 10 inches, and with that much color, I believe he is likely an adult. This is not my strongest knowledge area though, but he is definitely not a young juvenile. If he perks back up though, he will be a gorgeous lizard. I have always loved the orange coloring.
I hope some of that will be useful. Good luck! 
Elizabeth

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1.0 Mali Uro Archimedes (May he rest in peace)
0.0.1 Egyptian Uro Zuberi Mosca Khu (Mosca)
0.1 Sulcata Minnie
1.1 Iguanas Flik and Loki
Madison, Wisconsin