What have you increased the downloads to? Have you put a value in larger than your total source rasters plus overviews? Or just adjusted the rows and columns? The options are collected as columns and rows, so the columns and rows option should be greater than 19000,5335 and like I mentioned before increase the total number of number or records returned, the maximum number of rasters per mosaic and the max number of downloads. In my testing here, once I increased the values to exceed what was available in the mosaic dataset, It worked.
I am also assisting on your incident if you reply to that as well.
The only other question I have is if you are trying to get the entire raster into MicroStation, why not simply export the mosaic dataset and then add it there, rather than try to get them to download the export this way? The purpose of the image service is to make it more convenient not more problematic than pushing out the actual raster. The output from the REST page is just a new raster that is not in as fine a resolution as the original. If you are not consuming it over the internet, but consuming it locally, why create a new coarser version?
The download capabilities that you are modifying on the mosaic dataset are configured that way to allow some access to the underlying rasters via the web, but if you are trying to basically create a new raster from the web, there are easier ways.