If the first test comes back negative, would they likely do the confirmation test?
Nope. With DOT testing, a negative for all 5 panels on the initial screen means the testing is over as a "pass". Confirmation tests are only performed on any of the 5 panels that tested positive on the initial screen.

Also...

(a) As a laboratory you must consider the primary specimen to be dilute when:
(1) The creatinine concentration is greater than or equal to 2mg/dL but less than 20 mg/dL, and
(2) The specific gravity is greater than 1.0010 but less than 1.0030 on a single aliquot.

My question regarding those two is, do they test for significantly high levels of each (if possible), or are they only testing for low levels (dilution).
Labs look for low creatinine and SG levels in order to determine a "diluted" specimen. There are no upper limits established for creatinine. As long as the creatinine level is over 20 mg/dL, its satisfactory.

A urine sample is ruled "substituted" (aka "not consistent with human urine") when the creatinine level is less than 2 mg/dL, while the specific gravity is less than, or equal to 1.0010; or greater than 1.0200