The 2 feet below grade is to protect the pipe from shovels and other gardening implements more than anything else. If you are sure the pipe is not someplace where it might be damaged by anything, then a lesser depth should not be a problem. I know we have done designs where the pipe slopes from grade to some depth below grade in the past.
If the pipe is sloping down at all, then the end seal should be off of the pipe to ensure that no water will travel down into the end seal. This is just a typical electrical practice that we adhere to even though our end seal is rated for occasional submersion. Therefore steps should be made to elevated the end seal off of the pipe. If the pipe was doing a gradual rise, it would not be an issue.
For these calculations, plastic is plastic. The reason we care that it is plastic is because plastic is not a very good conductor of heat and therefore we have to derate the power output of the heating cable to compensate. If you said one part was metal and the rest was plastic, then we might have to make some adjustments. We always use our AT-180 fastening tape on plastic to increase the power output slightly and that is another reason why we need to know if the pipe is plastic; so the correct adhesive tape is selected. But whether the pipe is schedule 40, 80, or some other plastic all together, does not make enough of a difference for it to be a concern.