OBS off once runner demonstrates intent to advance?

[MTR]

No! Runner is at least returned to 3B or more likely, given your description that it was a close play at the plate even after having been obstructed, awarded home safely. If obstruction only gave a runner the next base, then you're encouraging F3 to obstruct on every extra base hit!

No, it is not possible to return the runner to 3rd. Either the umpire protected the runner to the plate or to 3rd. If to 3rd, then once the runner passes 3rd, the runner is in jeopardy. If protected to the plate, the runner is awarded home.

It is purely umpire judgment