ok to answer the questions after talking to an engineer myself..
highmount can be legal if it passes emissions testing as part of certification.
Bolt in and weld in half cages are legal only after being inspected and passing certification, it does not need to meet cams standards ie be cams approved to pass engineering buuuuuuuut why would u bother if it isnt!
all aftermarket coolers are actually illegal also unless engineered and securely attached with a certain grade of bolt through equivalent steel attached to the chasis of the car, lines and fittings are different to the cooler itself and will be tested (I dont know how, I presume pressure tested). rose jointed arms cant pass, coilovers cant pass unless u provide specific valving specs, brakes are as easy as doing a brake test and being bolted down with the correct grade bolt. Those are just for your common knowledge
interstate cars can be defected, the defect is specific to that state but the warning can also be transferred if it does not meet the states law in which the car is registered.