I found about this theater while researching other Detroit theaters. There was a "Duchess" theater on Grand River but this one (spelled "Dutchess") was on Jefferson Ave across from the Belle Isle Bridge. It was featured in an article about theater investment from 1912. You may note the "Mirth" in the article below looks damn close to the Dutchess in terms of appearance. Mike Rivest had the Mirth at 7327 E. Jefferson but I am going to assume for now that was actually the address for the Dutchess as it matches the location of being across from the Belle Isle Bridge. I found a classified ad for the Mirth which indicated an address of 1467 E Jefferson.
The following is the entry on the Mirth because they seem to be related.
This theater was across from Riverside Park on Jefferson according to the article below. There is a Riverside Park but it is off of Fort, not Jefferson. There was also a Riverview Park (now Gabriel Richard Park). Mike Rivest has an address of 7327 E Jefferson for this theater which is indeed near what is now Gabriel Richard Park.
The classified ad below indicates the address of this place was 1467 E. Jefferson. It is also very similar to the Dutchess if you look at the article below, which was supposedly near the Belle Isle Bridge, and again, near Gabriel Richard Park which was also Riverview Park (not Riverside Park). So it's all very confusing. To add to the confusion, the classified ad said go to 1415 Jefferson which is across the street from RIVERWALK PARK. UGGHHHHH!!!!!
Bottom line, I don't know where these joints were and I am a little suspicious about the "Dutchess" ever being completed.