Regarding the exchanges of the bodies. If one crosses and occupies the grey zone and moves the enemy lines (much) more often, one would collect way more dead off the battlefield. At the same time, Ukraine’s abilities to evacuate even their wounded from the “difficult”, as Zelensky calls them, parts of the front are very limited and often nonexistent. Also true for the Russian side, especially for those infiltrating behind the lines.
I would also propose that the Ukes do not bother much collecting the dead Russians, which is obvious from the very early exchanges when the territory did change hands and significant Russian casualties were reported, but the ratios would still be heavily in Russian favour in the exchanges. I highly doubt the practice has changed when they are constantly (on average) retreating.