The tech to lower Gaul mercenary cost is intended to provide a means for more sustainable merc production. You pay plenty of resources up front for long term savings.
Buffing the trade bonus doesn’t make sense because trade itself is not balanced for usual games. Buffing the trade bonus for carth means it will be very OP after any buffs to trade in general or when playing a map where trade is good.
the stone bonus is because of the amount of non-unit stone that the civ spends (temples, walls, big houses, etc). Historically, I’m sure it took plenty of stone to build Carthage so I don’t think its a problem historically.