USAT rules limit thickness to 5mil. You don't want to have that 5mil all over your body, however. Most tri wetsuits have 5mil on the chest, back and legs and use 2-3 mil on the shoulders and arms for flexibility. The exact layout of thicknesses varies from suit to suit.

Scuba wetsuits are different than tri wetsuits. Scuba suits have fabric on the outside to provide durability and abrasion resistance. Tri suits have bare rubber on the outside because it's hydrophobic and lets you slide quickly through the water. Also, a tri wetsuit will have a much more snug fit than a scuba wetsuit.

A scuba wetsuit will keep you warmer on the swim, but in 70° water, depending on your tolerance, it might not be worth it. Either way, the 6mil suit is against the rules.

6mil suit is against the rules as it is mentioned above.
Don´t know how they could measure everyones suit....but it´s against the rules.

Regarding temperature in USAT and ITU rules (that are available in their sites) the allowed temperatures for wet suit usage are listed.