I've probably missed some things, but why doesn't the controlling government authority want the jetty rocks off what looks like a public right-of-way/street? As others wrote, the restricted with can be a safety hazard.
Yes, I (and probably others) have said something like that too.
I don't understand getting a lawyer involved, spending that money, for what some govt agency should be doing for you for free.
The heck with moving part of it 5' and part 2', get the whole thing off that 20' access. There might even be an easement alongside that, with no building allowed in the easement. We have a 10' easement alongside each side of the road at our house.
You are tip toeing around this. Get the controlling municipality involved, the sooner the better. And you should make sure your stuff is in compliance as well, looks like your sheds weren't/aren't?
I don't think you ever told us how long the 'jetty' has been there - you did say he built it all in a day or two. When was that?
edit/add: Plus, if you settle for a compromise, and it goes on for years, you are going to fall into that "adverse possession" area of the law, and you may never get your 20' back. Could affect resale value of the house when potential new owners see they were supposed to have a 20' wide access, and they don't.
Unless I'm missing something, I don't think that lawyer is doing you good service. If he can make a few hundred getting a compromise, it was easy work. It's not his driveway.
-ERD50