73 & 74 Proof, Same Working Die?

Can't tell much of anything from the images without a description of what you are seeing for each one. They all look like big blobs.

It is very possible that your two coins could have been struck with the same reverse die, but more likely that the same master die or working hub might have een used for both. There's no telling which is which, though.

I don't believe the line to be re-engraved - it appears to be a design characteristic that would be on all coins.

There probably isn't any real 'die wear' on either coin. Proofs do not commonly have die wear.

The lines between the legs appear to be, again, part of the design.

When attributing proof coins to be from the same die, the small pock marks made by perpendicular grains of metal in the fields are what you should look for. These appear as small die flow lines with 'grains' in them. Another thing to look for is very tiny, thin die scratches. Yet another good marker is small die dots on the devices. These types of things are usually a die to die thing.

Re-engraved devices, if they exist, are usually done on the master, which is likely used from year to year and bears no real consequence. In other words, it's common and well known. Exact placement, size and shape of devices tends to change only every few years.