Welcome.
Your friend's analysis made me laugh-- indeed, any or all of the things your friend mentioned might
[/i][i] be problems. But you are still left with figuring out which is which doing what.
I am completely ignorant about gardening in an LV type of climate. But I can offer some suggestions. First, you are describing symptoms. Have you spent some time looking carefully at the affected leaves and plants? So for example, the curling of the tomato leaves might be a symptom of an aphid infestation. The holes on the holes in the leaves suggest caterpillars, or possibly beetles. Have you turned over some leaves-- do you see aphids? Have you gone out in the morning and inspected for caterpillars? What's flying around your tomatoes (would be a clue to what kind of larvae are present)? Point is-- you need to stick your head in there and look for things (bugs, etc.) that might be the culprits. The same would apply to yellowing squash leaves. Just the leaves or the leaves and stems? Does there seem to be anything
growing on the leaves (like a fungus or mildew)?
Next suggestion-- find the Nevada Ag Extension site. Most states have gobbs of stuff on-line to help home gardeners identify common pests and problems. You can use this information in conjunction with closer inspection of your plants-- it might suggest particular things to look for, as well as how to control the problem.
Sorry I can't give more specific help. You might want to post some pictures of the symptoms on this web site. A picture is worth a lot here!
Oh! If you prepared the soil well, it's highly unlikely that it N-deficiency. Of course, you do live in a desert, so I could be wrong.
Good luck!