I have mostly boated in the NW so the situation may be different on the San Juan.
1. Are you required to remove the boat?
No, but someone else will retrieve it if it can be done.
2. If someone else manages to free the boat, do they now own it under the maritime salvage act? How would this apply to a boat that got loose and drifted off?
Not in Oregon. There is no marine salvage law. The boat still belongs to the original owner.
3. On a permitted river how do you arrange a second trip to try and retrieve it?
Through the usual permit process. Why didn't you have the equipment to get it the first time?
4. Will the Forest Service or BLM help with a salvage effort?
Not on rivers such as the Middle Fork Salmon and Rogue unless they happen to be there, although the Park Service has sent Rangers and helicopters to retrieve boats on the Grand.
Real adventure is only one bad decision away.