I've read enough of your posts to respect your opinion, but:
1. I do not like the 5DIV above f/11; the loss of resolution is excessive. In fact, at some point I'll pull out my 6D to compare, f/11-f/16.
2. The 80D resolution is distinctly inferior for macro, with the same lens (Sigma 70mm Art) and framing the subject the same.
Is it likely I have a bad copy of the 80D? What does DLA mean to you?
1. Diffraction is a function of enlargement, if you enlarge the 5D MkIV and 6D the diffraction is the same, the smaller pixels of the 5D MkIV might be able to define the diffraction better, but the diffraction is the same in both images at the same size. It's only when you go to "100%" (or any percentage enlargement of pixel numbers not area) that the differences in same sized sensors becomes visible, but think about it, if you enlarge anything 'more' than something else it will show optical aberrations more even though they are actually the same.
2. Probably not, but you are enlarging a smaller pixel to a larger size thus exaggerating any aberrations more.
Possible, but probably not. DLA means this to me
https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
Imagine this scenario. You have a 24mm long fly, you can reproduce that at 1:1 on your 80D, 5D MkIV or 6D, on the 80D it fills the frame but it is reproduced the same size on all the sensors. Take the three shots at the same settings.
Now make three 8" x 10" prints where the fly fills the paper, the 80D print is uncropped, obviously, the other two are cropped but the fly is the same size on all three prints. The diffraction is the same in all three prints. Now really crop in to the eye of the fly and make three more 8" x 10" prints where just the eye of the fly fills the print, the diffraction is the same but, assuming good technique etc the image with the most pixels will retain the most detail even though theoretically it is 'suffering' DLA, all it really means is the pixels are capable of defining the airy discs of the diffraction, those airy discs are the same in all three photos though.
More pixels on a same area are never worse, they just sometimes don't give you much more detail, especially in macro shooting scenarios where reproduction size and capture ratios are so important.