Shades of the Texas case
Welcome to the middle of the infamous and always-heated "us vs. the archies" (archies=archaeologists) and "us vs. the public land laws" arguments that have been raging -- sometimes violently, post-wise -- for decades in this hobby. Long story short, the law always wins.
Yes, we ALL would do whatever we can to help preserve history and donate hundreds of hours of our time and detecting expertise to help archaeologists and museum people find, recover and preserve local/national history. In a lot of respects, an appreciation for local history is why we got interested in detecting in the first place. Sometimes they welcome our help, but most times they don't. But the key here is when in sensitive areas, you have to go by *their* agenda and needs, not yours or this hobby's -- even if it's clear that they're leaving great artifacts that would contribute to this history left to rot in the ground, or have no interest at all for whatever reasons they have. Those e-mails that suddenly stopped was a clue to that lack of interest.
This reminds me of a HUGE case -- debated endlessly and heatedly in a different forum elsewhere -- about 6 or 8 years ago when two detector guys were caught diving in a lake around a long-lost (but nonetheless federally protected) sunken Civil War fort in Texas. It got REALLY ugly beyond even Civil War standards and really, quite embarrassing for everyone who debated the whole thing, myself included. What happened to you (detectors confiscated, fines and citations, court date, etc.) was *exactly* the same thing that happened to them, in the same order of events, even down to the rangers' ambush and then handling it in the exact-same sh!tty way. They *knew* there was once a fort there and were intentionally hunting outside the original perimeter (and therefore well outside protected land) for relics if I recall right, but that case revolved around a major question of where the boundaries actually were then and where they today and whether these guys actually *were* hunting on federally-protected land because, well, the government couldn't seem to make up its mind on where the fort actually is now.
This went on for a good 2 years of court dates and hearings and depositions and all sorts of discomfort for the 2 detecting guys because they decided there was a principle here and they were 100% right and the government sucks and blahblahblah. Needless to say, in the end they lost decisively and ended up spending many thousands of dollars of their own money in the process -- along with a good sum in donations from sympathetic forum individuals.
Mostly tho, when all is said and done, it's a federal offense to detect anywhere along a National Seashore for any reason. Working approved and onsite with archies on a project is about the only way around that one, but clearly you weren't. If it was me in your situation, I wouldn't fight it because clearly you were in violation of the law no matter how noble your intentions, and appeal to the judge with your story because, on the bright side, it isn't likely you'll end up in prison or anything -- consider it a giant lesson learned. Just don't hold it too much against the law like a lot of other people have. They were doing their job as the law demands they do (altho I agree they definitely SHOULD have treated you better) -- and as a law enforcement officer, I'm sure you more than any of us can appreciate that.