. . . so any boundary created from the 'lines' will not replicate the original boundary.
You're right the 'boundary' will follow up to the extents of the limits the lines had, so most of the time will be chamfered
Is the original surrounding geometry still available in the drawing ?
No, as in the problem I had when I had to solve it (like in january of this year) there is no known boundary, just supposed. I created new ones and deleted them just for the examples. but they're all irregular shapes anyone can create.