I don’t think there is an ADA issue - Disneyland just did major work to the treehouse and reopened it. Removing the bridge to the larger island and redoing a path doesn’t make this a “new build”
That's not even the whole bottom island. Imo that's not enough to be worth saving, but that's just my opinion. Plus, your map ignores the Haunted Mansion queue. Which means another path would need to be added. Which means cutting into the river. Which would either be skinny enough to be a bottleneck or wide enough to effect the riverboat.
Of course, the queue and entrance to Haunted Mansion could be redone completely to open that up having guests enter from behind in Villains land. But that seems like a lot of effort just to save 1/4 of the riverboat imo. Especially the least interesting 1/4