There is another element which you touched on, which is hydraulic pressure. Dog sex is not the same as human sex. We can see that the vagina is a pressurized environment when a woman is knotted by the force with which the sperm is expelled when a knot is pulled out.
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There are even more impressive examples.
The question is then whether some significant amount of sperm would enter the uterus under pressure if the os is at maximum normal dilation.
The second question is what the anatomy of the vagina looks like when it contains a large dog penis and knot. Presumably it will not have the same appearance at it would at rest, and would more likely look like a sleeve wrapped around the dick, with all the curves and carnies straightened out.
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Having a scientific mind, there is an easy way to test the first - use a cheap straight endoscope (around $30 these days) or a speculum and examine the cervix after a good knotting session - if sperm is running from the cervix some must have gotten in there.
The second can be tested with a large transparent dildo and another endoscopy.