mansianand1234
is it right to say that 'yet' is a parallelism marker?
Yes.
In this problem, there are two different 'levels' of parallelism.
• At larger scale, the two main parallel structures—which form a contrast—are
is not only completely man-made but also designed exclusively for human useand
is classified as a state park.
The contrast is properly indicated with "yet". (These two phrases contrast because the primary purpose of state and national parks is to keep those areas undisturbed in their natural state.)
• The first of those two larger-scale structures is itself a parallel construction, with "completely man-made" and "designed exclusively for human use"—two ideas that both reinforce the point that the G-H Trail is decidedly NOT 'natural'—as parallel parts.