Can anyone tell me why other breakpoints that were perfect have now become out of position just because I added another breakpoint between them?
you answered your own question
I have a large site I created in Muse, containing around 90 pages/screens, with 6 fixed breakpoints
I sincerely and deeply sympathize with you. And also sincerely admire. Muse is not designed for such a large number of pages the site. You managed to dig up the corn field with the help of a shovel child. I think, now you are experiencing considerable difficulties with the administration of this site, is not it?