I can say that I have never hunted in the "field trial area" and would not do so, as that would be in bad form, even though it's closer to me than some places I hunt. And I would be very surprised if other field trial participants did so. There are plenty of other covers for grouse and woodcock in that part of PA.
However, it is public land, so I reckon people who are not involved in trialing hunt it. As is their legal right. I don't go over there during hunting season, so if there are people hunting around there, I very much doubt it's anyone who discovered it via trialing.
By the time trialing begins in September, both grouse and woodcock are big enough to fly, and the trialing season ends before nesting. No harm done. If I felt like it was harming anything, I wouldn't take part.
As far as pressure on grouse.... I'd say that most of us who hunt grouse realize it's a delicate resource that can fluctuate year to year. Once I shoot a bird, I am done in that cover for the year, period. And I will only hunt a cover once or twice a season. I took a couple new people out this year, and I told them why we approach it that way, that it's not a stocked pheasant field. Personally I am pretty much done shooting woodcock period, except to reward a young dog who has handled perfectly & needs feather in its mouth. I did shoot a couple this year. They are worth more to me on the wing than on the plate.
In terms of grouse hunting, I hope we get more hunters (to advocate for the resource) and more habitat. Indeed, our local RGS chapter is meeting with PGC biologists next week to help create some woodcock habitat locally, and we're planning to help run an event for new handlers/hunters in April, w/ focus on habitat, scouting, and dog basics.