Hi NightCabbage - here's a bit of an illustration on how that area can be repaired by building a surface that touches the others with better accuracy.

To start with, I duplicated this edge at the bottom, by selecting it and then doing Copy and then Paste:



Now select the inaccurate face here and just delete it:



Now to make a better surface there, select these 4 curves - note that 3 of them are edges of the other surfaces:



And then run Construct > Network to build a surface from them. The generated surface will be within a good tolerance of those existing edges, so you can then select those surfaces and use Edit > Join to glue them together and get a proper single joined edge in that spot.


Once you have that set up, you can then select that edge:



And then Chamfer will work on it since it is a good single joined edge instead of it being 2 unattached edges with a gap between them:




That's the kind of sequence that can be used to repair something like this that has some pieces that are unjoinable because they are too far apart.


- Michael