First hand data points:

When all flights are booked on single PNR, Jet will at least honor baggage policy of originating airline as per USDOT rules if the first segment originates in US. If it originates in Europe Jet usually follows IATA's MDC (most dominant carrier) rules.

In both cases, stopovers dont affect the allowance (this was a concern for us but Jet did not create any issues)

Now of course, Jet wil allow more allowance than what USDOt/MDC rules allow if it is customery to do so for that segment and class for Jet.

Also, if something is specifically included/excluded on its website, that supercedes everything (advisable to take printouts with you)