I took the liberty to enlarge some problem sections of the $50 note in question. I have seen enough doctored notes that these spots jumped out at me without enlargement or zoomed images. So I thought it would be beneficial to take a few screen captures when enlarging just to point out the suspected regions of interest. Creases are obvious through the president's face/seals but suddenly disappear on the white paper regions. Personally, I find smudgy looking borders to be the 1st big "red flag" & then I start to magnify as much as I can:
The first section as described in the OP shows where eraser (or some other abrasive) has been used near the blue seal. "Series of 1914" where the "f' & "4" have nearly been obliterated. Parts of the inner border seem too faint:

Below is a vertical region which looks to be erased or doctored where there was once a typically darker crease. Note how the left bottom curl of the "F" (Frank White's) signature has been lightened:

There is a portion in front of the ship's bow that looks like it might have had rust or some stain removed as well (in between the computer > guides):

I agree with the feedback that when you're looking at notes of this price range, you're better off selecting a trusted
TPG certified note. Still inspect it as you would any other note (certifiers are imperfect too), but then proceed slowly (& with caution).