The only reference I can find with an Do-217E in a Mistel configuration was with the Me-328 program. According to Mistel - German Composite Aircraft and Operations 1942-1945 by Robert Forsyth, Me-328 V1 (without engines) was mounted on Do-217E (JT-FL) and tested in Horsching in the autumn of 1943. The other test with this and the DFS-228 were done on the K version.
Structural wise I think you are correct, it's of a carrier aircraft. The mounting shown in your pic is about the thickness used in testing the aircraft you mentioned. The ones used to carry Mistel bombs were a lot thicker.