I can't find anything regarding that it matters if the ticket is bought in Europe? what I do know for certain is that flights ending in the USA the origin market/carriers baggage policy applies.
most significant carrier applies to when you have to re-cehck in. To be precise, it is not the origin carrier, but the marketing carrier of the first segment. In this case, that appears to be JU. Thus, the JU allowance will apply to all segments of the ticket out to CMH and back (if there is a return booked on the ticket).
the link I posted is really in depth about the comparison of the 2 rules. anyways I hate that I used to last post of the day for this
used ita matrix and scanner to find
BEG - CDG - CVG
80 dollars more expensive than yesterday but it does use Air Serbia for Beograd to Paris