These photos from a 2015 bike perhaps give a better idea of the problem
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Looking at the bottom picture it does look like you could possibly lengthen the bottom of the mudguard visible on the photo, which may help prevent the tyre spray on the mount from the back wheel.
In regard to the first photo -
It tends to indicate that the majority of the mud on that side is thrown up by the front wheel, so perhaps the additional mudflap on the front engine plate (post #1) may lessen the spray from the front wheel, if it’s hitting the sump it has to be spraying the rear shock too.