pacem would be the accusative[0] vs the ablative[1] - the difference being him being peaceful in rest, vs his rest being peaceful.
Nonetheless, I echo your sentiment.
[0] https://en.wikipedia.org/wiki/Accusative_case
[1] https://en.wikipedia.org/wiki/Ablative_case
"Requiet in pacem" is what I usually see on teh interwebz, which proves absolutely nothing.
FWIW I quit Latin in 10th grade.
pacem would be the accusative[0] vs the ablative[1] - the difference being him being peaceful in rest, vs his rest being peaceful.
Nonetheless, I echo your sentiment.
[0] https://en.wikipedia.org/wiki/Accusative_case
[1] https://en.wikipedia.org/wiki/Ablative_case