BillFlowerpot You'd definitely need a mod to change the location merchants' working hours. I don't know whether such a mod exists, but I can't find any at least. You could ask on the NRaas site instead if you want a more definitive answer.
As for the merchants getting stuck, they're supposed to stand there and not do much most of the time. Is the problem that you can't buy anything from them, or something else? You could always try an NRaas DebugEnabler reset on the sim in question just before you want your own sim to buy something—this clears the action queue and should free them up to interact with your sim. But if the problem is just that they're not doing much of anything, that's by design.