Re: Do we need a structural engineer?
A single room in a 20x25 house would not have been an original floorplan in 1850. If bearing walls have been removed failure is inevitable. One of our specialties has become effecting "structural remediations" guided by a very competent and historically sensitive engineer. You don't want your efforts to essentially be poking around the edges, if something needs to be done, you want to do it all at once, and with the best advice, and with a safe plan. If it's as bad as it could be, it will be a very disruptive job.
During demo/discovery, the original floorplan will likely be revealed, if a set of trained eyes is there to interpret the vestiges of walls and doorways. There could be a literal roadmap under that 1970's parquet.
The 1818 brick house we saved in 2009 had been "opened up" in the teens or 20's, so the new point loads they created took quite a few decades to end up with the 7" sag we encountered. But t was still moving down, down. You may have decades of service left, or not.
I don't know why you would get cracks at all windows from a floor beam/support issue. I take it this is a wood-frame house?
Remove not the ancient landmark, which your fathers have set.