I had floors refinished and professional didn’t get all old stain off in corners and under cabinets before he restained and put on poly. I had stain color changed so it looks obvious. He said he will come and fix it but can it be blended in evenly with the rest of the floor stain and finish.

It takes a floor mechanic to get ready for a stain, using the right tools. It could take an artist to blend mistakes after the floor has a coat of finish applied. Some people can use paint to get a match. They are very talented. It is much arrester to get the stain to look right after sanding. The finish protects the color.

Floor guy came today to try and correct mistakes. Looks worse than before. Is the only solution to redo the whole thing. He definitely doesn’t know how to fix it so far.

The floor guy may still pull it off. He will be learning how it needs to be done.
I don't think that he is bothering the floor even if he needs to sand the whole thing again if it has to be done.
I think that he needs to decide when to stop and start over.