A good number of dogs with cushing's have a urinary tract infection at the time of diagnosis and yes, white blood cells are found in the urine. Cushdogs usually have dilute urine which reduces the number of bacteria in a specimen so a routine urinalysis doesn't pick up the true number of white blood cells. For this reason, a urine culture should be done to identify the bacteria so an appropriate antibiotic can be prescribed.
A blood chemistry often includes T4 which is a thyroid hormone. Can you check your copy of the blood tests to see if T4 is listed? If so, can you please post the result as well as the normal reference range?
Most specialist and experienced gp vets prefer the LDDS test over the acth stim test for two reasons. The acth stim tests is more likely to yield a false negative result so a fair number of dogs with cushing's, especially a dog with an adrenal tumor, will have a normal result. The LDDS can often tell you that a dog has pituitary dependent cushing's, whereas the acth stim test doesn't, which requires additional differentiation test like an abdominal ultrasound or an endogenous acth test.