Oh Elaine! 
You sound so stressed! I am not sure this will help. First of all, the 55 gallon will work for the moment, but they are so narrow they really aren't ideal, even at this size. The good news is that they do allow for a good temperature gradient which is awesome!
In general under tank heaters are frowned on, and I don't remember exactly why, I do know that it is mostly ambient and basking temps that are important. Your temps look good, a basking spot of 115-125 is ideal, with a gradient down to the uppers 80s or below. For juveniles we usually recommend you keep them on paper until they get a little older, but a 5" svl leads me to believe it *could* be a year old, which makes the sand ok. You are on the right track with the crickets, only very sparingly if at all. Many of us on the forum have stopped feeding them altogether. Your thought process on the water seems to be on track too. If he is looking a little dehydrated you can also give him a good soak. They tend to not be overly find of the process, but it is good for them on occasion, just make sure that the water is hot (but not too hot to touch obviously - I use the baby test on the sensitive part of my wrist). Soak him for 5-10 minutes, making sure to keep his head up, they aren't real bright about that sort of thing. You also want to make sure that he has at least 2 hides, one on the cool and one on the warm side, I don't know if snakes are the same way but Uros like low, tight spaces, as opposed to very roomy ones.
As far as the UV goes, it is imperative that they get UV, but I am not sure that I have ever seen an actual number on it. I read somewhere eons ago that 10 minutes in unfiltered sunlight (outside) daily would meet their needs... but pet shops were also touting crushed walnut shells as a good substrate then.. so who knows. Keeping a good UVB light within 12 inches of where the lizard spends most of his time should ensure plenty. SOme people use supplements, which you have to be careful with so that they don't get too much. Others use mercury vapor bulbs, which I dislike due to their mercury content, but some people really like.
What sort of veggies are you feeding? Greens should make up the staple of his diet, with other things sprinkled on. You should feed a good multivitamin and calcium supplement a few times a week as well (I use the herptivite and corresponding calcium supplement myself). I have a 16 month old Egyptian, and my breeder recommended feeding pretty bird finch food occasionally (maybe once a week). It is the brightly colored, little extruded bird food, smells really good. Then once the Egyptian gets older switching them to juvenile iguana pellets (once again, just occasionally).
There are many threads on good soil substrates in this forum, so I won't go into that. I use sand myself, but do wear a mask when cleaning the tank.
Now, for the big thing. There is a big difference between Malis and Egyptians, and its SIZE. There are two types of Egyptians, microlepsis, and aegypticus. The microlepsis tend to be smaller (about 2ft adult size), the Aegypticus can get up to 3ft long. These are BIG lizards. I have my 16 month old in a cage that is 2ft by 4ft, and I expect I will need to replace that tank within a year or two for something bigger. They tend to have pretty laid back personalities once they get a little older, but they do take up space. Here is a good resource on the Egyptians:
http://www.deerfernfarms.com/Uromastyx_Egyptian.htm
That entire site is an excellent resource for Uro care.
The good news is that you never have to microwave dead animals to feed to the Uros
.
It sounds like you have a much better idea of what it takes to care for these guys than most when they get them, which is fabulous, and personally I think they are the greatest pet ever. I had a mali for almost 10 years, that I just had to put down this past summer, and I would not have traded him for anything else.
There are a lot of extremely knowledgable people on this board, who I am sure will weigh in as well. Keep asking questions, and welcome to the board! We also love to see pictures, so if you have a digital camera throw a few up!

Elizabeth

-----
1.0 Mali Uro Archimedes (May he rest in peace)
0.0.1 Egyptian Uro Zuberi Mosca Khu (Mosca)
0.0 Fish
0.1 Sulcata Minnie
1.1 Iguanas Flik and Loki
0.1 Newfoundland Jasmine (RIP)
0.1 Feline Winter
Indiana & Wisconsin