Those windows are toast. Your least expensive option would be a vinyl pocket replacement window. That is a replacement window that goes into the existing frame of the window.
Is the frame of the window rotted as well?
That's what realtors call charm. :rolleyes: But seriously that looks like just settling over the years. I have a fireplace that has the exact same tile for the heart and it has settled and looks pretty much just like your fireplace.
It's hard to say if there's any structural issues without...
Not sure what "A36" Steel is but what steel guys have recommended to me in the past is 1/4" thick steel. If you are worried about corrosion just clean up the plate and prime it with a good steel primer and you should be fine.
You want to shore up what you think may be sagging, adjustable jacks are pretty inexpensive and would do the trick most likely.
Kind of hard to say for sure without some good pics showing the issue.