Have you talked with the people at your county office of the University of Minnesota cooperative Extension Service about this? Since the onset apparently was fairly sudden I would suspect a plant poison. That grass as well as plants were involved points to that as well. Testing would confirm that but may be fairly expensive. Here is a link that might be useful: UM CES

We have the exact situation. We removed the grass, dug out more of the lawn, about three inches wide and placed a form flush with the grass. There will be about a six to eight inch trough depth between the pavers and the form. We used rocks or rubble and poured dry cement between the rocks and sprayed the cement with water. We let it dry and repeated layer by layer for about five inches, The last three inches we mixed the cement and poured the final layers to the level of the pavers. We added a few decorative rocks in the cement for effect. We left the forms in place. The repair was approximately twenty feet long. The result is the grass edge is the same depth as the rest of the lawn. Nice green edges!

most grasses are full sun ... and what tree is above.. some of the maple clan.. among others.. simply suck the life out of plants under ... are we to presume these are garden grasses.. rather than lawn plugs??? where are you.. and are they zone appropriate.. could winter have set them back??? ken

Also I should add, in the 3rd pic the dead rectangular patch was from me.. I had something sitting on the grass for a couple hours and ended up frying the grass a bit. Also before rolling out the new sod last summer I rented a rototiller and rototillerd my entire back yard then leveled it all out beforehand.