Since the above code does only attribute kills to player 1 in singleplayer when playing cooperative multiplayer kills get "lost" to infighting and indirect kills (monsters shooting barrels).

This affects the both level stats and tally screen when doing UV Max co-op runs - making them a little unpleasant QoL-wise.

It would be preferable to have kills always be attributed to a player (or have an option to at least force this behavior). What's your thoughts about this?

I don't know how original executables attributed those.
PrBoom+/DSDA-Doom attribute kills to the last enemy of the dying object if it was a player or the first player who is still ingame otherwise.

https://github.com/coelckers/prboom-plus/blob/e08f129f557cae6e529d8f9ff151750a2345a16a/prboom2/src/p_inter.c#L704-L723

Demo (solo-net) showing the behavior: infight-tally-00000.lmp (2:18 min, all kills @ 1:30)